What's more important, testosterone or nitric oxide? I couldn't answer that one and I doubt you can either - both of these molecules just impact too many functions and systems to declare a clear victor. What is important to note, though, is the INTERACTION of these two on each other and how they partner to improve men's health in, literally, a hundred different ways. Both of these have been found by multiple studies to impact endothelial function, i.e. nitric oxide and blood flow, sexual health, brain function, sleep, mood, exercise and athletic performance and so on. Yes, they affect almost everything we hold dear as males.
Let's start with testosterone and its role in protecting nitric oxide. Having ample testosterone levels is a key, researchers have found, to maintaining arterial health. I cover this in my page on Testosterone and Heart Disease. The quick summary is that testosterone actually optimizes and even boosts nitric oxide levels through action on eNOS, the same enzyme that Viagra works on. Testosterone also lowers arterial inflammation according to a couple of studies.  Its actions are so important to your cardiovascular system that low testosterone levels have been shown to be a significant risk factor for many chronic cardiovascular diseases, a fact I will go into in more detail below. And this is one of the things I found so exasperating when doctors are unwilling to treat low testosterone: they put their male patients at risk for many nasty medical conditions, especially heart disease.
And then let's examine the most glaring example: low testosterone tends to accelerate hardening of the arteries and arteriosclerosis. And these conditions can definitely affect your nitric oxide levels in the medium and long terms. How? Simple: arteries covered with plaque cannot produce sufficient nitric oxide for things like erections and safe blood pressure levels. Of course, there are other problems with hardening and narrowing of the arteries - like a heart attack or stroke! - but this limiting of nitric oxide output is one of the symptoms that a man will generally first notice. As many of you know, it first shows up often as erectile dysfunction. And it is very common in the U.S. for a man in his 40's or 50's to begin to experience the beginnings of prehypertension/hypertension as well.
But the point is this: low testosterone has been shown to accelerate arterial decline, which in turn helps lead to a decrease in nitric oxide levels. So to one degree or another, your endothelial nitric oxide may be dependent on your testosterone levels.
All of this explains why so often on The Peak Testosterone Forum, I see men with BOTH testosterone and nitric oxide related issues. They likely tend to snowball downward together. And I believe this is shown in the studies, where a significant percentage of men given HRT (testosterone therapy) are not cured of their erectile dysfunction, a subject that I cover in my link on Viagra Failure.
Likewise, HRT often fails to cure erectile dysfunction on its own. One meta-analysis, where researchers aggregate or pool studies together, showed that about 57% of patients with erectile dysfunction responded to testosterone therapy.  Of course, that implies that in about 43% of patients, HRT did not really help. NOTE: My guess is that there would probably have a higher response rate if it was properly managed, i.e. testosterone levels were raised significantly and estradiol levels were managed as well.
So testosterone, in particular low testosterone, can affect nitric oxide. What about the other way around? Well, let's look at what I call The Three Main Natural Engines of Testosterone Production. In all three cases, I will show that low nitric oxide levels can negatively impact every one of them:
1. Sleep. Nitric oxide is VERY important to sleep and research has shown that it is an actual regulator of your sleep/wake cycles.  It likely does this by altering the release of the neurotransmitter acetylcholine in the brain. To test the impact of NO on sleep, researchers gave rabbits various nitric oxide inhibitors and found that it lowered REM eye movements.  And a study on rats showed that nitric oxide inhibitors actually decreased the amount of time in sleep.  Sleep researchers have noted that decreased NO will actually alter EEGs and ages the brain in several key ways. 
2. Sexual Intercourse. Of course, sexual intercourse and nitric oxide go hand in hand and Viagra proved that forever by putting million of men "back in the game". Viagra works, of course, by slowing the breakdown of endothelial nitric oxide, allowing levels to build up more quickly. One study found that both Viagra and Cialis increased the frequency of sexual intercourse in men by a little over 40%.  One study of Cialis usage showed that it did, indeed, give a modest increase in testosterone and a nice boost to the T/E2 ratio. See my link on The Testosterone-to-Estradiol Ratio for more information.
3. Mood and Attitude. I document many cases where the mind and Testosterone are very related. Competition and victory can very substantially raise testosterone. Depression can lower it Once again, this ties in with the fact that low nitric oxide levels can definitely affect mood. In its extreme form, it has been implicated as being a factor at times in clinical depression and schizophrenia.  For more information, see my links on Testosterone and the Mind and Testosterone and Depression for more information.
So, although I do not have a study to prove it, it is very likely that low nitric oxide can lower testosterone and low testosterone can lower nitric oxide levels over time. Clearly this could create a viscous circle and is part of the explanation as to how men living a Western lifestyle spiral downard in health terms so rapidly after about the age of 40 or 45.
The good news? There is hope if you're willing to go natural, because there are natural ways to actually reverse, or at least significantly partially reverse these simultaneous decreases in nitric oxide and testosterone. Here are some examples:
1. Testosterone. There are literally dozens of ways to naturally reverse some of the smaller losses in testosterone through lifestyle changes and I have outlined these in my link on How to Increase Your Testosterone Naturally. Now by far the biggest of these is weight loss. Men who lose massive amounts of weight can in some cases literally more than double their testosterone levels. Some men, though, simply cannot increase the testosterone enough through lifestyle changes alone and will have to consider HRT (Hormone Replacement Therapy). However, in my opinion if this is well-managed and well-monitored and the patient is brought to normal levels of testosterone, then this can still be considered a natural solution. The reason is that one is simply replacing testosterone that is missing from normal aging or from damage.
2. Nitric Oxide. Again, there are dozens of ways to boost your nitric oxide. One is through consumption of high nitrate foods, something I discuss in My Review of Beet the Odds . (This methodology is ultracritical for you 40+ men.) Still another is through following the principles of my book, The Peak Erectile Strength Diet , which is all about foods that will increase nitric oxide and blood flow. If you go through the links listed here in my Improving Your Erectile Strength section, you will come across many other strategies as well.
I should also add that you can heal you arteries by actually reversing some of that arterial plaque. I cover this in my link on How to Clear Your Arteries.
The important thing is that if you raise testosterone, you will also likely raise nitric oxide levels. And, if you raise nitric oxide, you will be improving sleep, the frequency of sexual intercourse and mood, all of which will very likely boost testosterone. Yes, you should be able to not only halt the decline, but actually turn back the clock significantly.
1) The Journal of Urology, Aug 2000, 164(2):371-375, "TESTOSTERONE SUPPLEMENTATION FOR ERECTILE DYSFUNCTION: RESULTS OF A META-ANALYSIS"
2) The Journal of Neuroscience, 15 Jan 1997, 17(2):774-785, "Pontine Nitric Oxide Modulates Acetylcholine Release, Rapid Eye Movement Sleep Generation, and Respiratory Rate"
3) AJP - Regu Physiol, Jan 1, 1994, 266(1):R151-R157, "Inhibition of nitric oxide synthesis suppresses sleep in rabbits"
4) Brain Research, Jan 1999, 816(1):209ï¿½219, "Endogenous nitric oxide in the rat pons promotes sleep"
5) Sleep Medicine Reviews, Apr 2005, 9(2):101ï¿½113, "Nitric Oxide and sleep"
6) BJU International, Dec 2005, 96(9):1323ï¿½1332, "An open-label, multicentre, randomized, crossover study comparing sildenafil citrate and tadalafil for treating erectile dysfunction in men naï¿½ve to phosphodiesterase 5 inhibitor therapy"
7) Neuroscience, Jan 1998, 83(3):867ï¿½875, "Nitric oxide synthase-containing neurons in the human hypothalamus: reduced number of immunoreactive cells in the paraventricular nucleus of depressive patients and schizophrenics"
8) Mol. Hum. Reprod, 2010, 16(10):761-769, "Endothelial regulation of eNOS, PAI-1 and t-PA by testosterone and dihydrotestosterone in vitro and in vivo"