I was 99% sold on a fixed, but then I thought how long I will keep this CO2 setup (most likely forever) and I got the whole setup from formattfab on this forum, and I am amazed how many times I have "turned it down".
I vote spend the bucks for adj. It has the best of both worlds, and there is no reason I can think of not to get it.... except of course its a few bucks more, but in five years when you wish you could lower it, that won't matter.
One of the goofiest reason's I have lowered it is to pump up my kids flat bike tires, with 150, the stem will go to deep into the rim and the bike stem won't push back enough to get CO2 to start coming out, but at 50 starts no problem, then bump it up.
I'm running a fixed regulator at 120 psi to run tools and air up my tools. I chose a fixed regulator so the tank, which only gets used out on the trail, won't fall over and damage an expensive adjustable regulator. Fixed regulators are cheap and durable and I've got an air compressor for doing things at home that require lower pressure.
It depends on the regulator. The cheaper ones made for beer and soda service can't handle very high flow rates. They can freeze up if you're filling 4 big tires quickly. Slow it down, though, and they'll work okay.