By warm temperatures, do you mean +100*F? And how would each compare in 0-15*F temperatures?
Oh, two more questions I forgot. If I were to install an on/ off co2 tank valve, would that need to be done professionally? And I remember you talking about a type of oring that is best for cold temperatures, which type is that again? And is there a company that makes tanks orings with that material?
The temperature that CO2 goes supercritical is about 90 degrees. The severity of the problem depends on the setup. I devolumize the crap out of my setups, and I've never had a problem with having to intentionally prime my gun, even in 95 degree weather. The more volume you have in your setup, the more noticeable the problem. Again, in cold temperatures, the performance depends on the particular setup. In a regulated setup, the higher your operating pressure, the more risk you have of the CO2 dropping in pressure below the output of your regulator. If your operating pressure is 200 PSI, it's not a problem, but if your operating pressure is 600-800, things probably won't turn out so good. Siphon setups have no problem with the cold.
As for installing a valve, it depends on how much confidence you have in your technical abilities. I do all mine myself with just a vice and a strap wrench. As for O-rings, there are tons of different types of materials that work well in the cold. I've found that MILSPEC Buna N works very well, and is still pretty cheap. I get all mine from Mcmastercarr.com. If you are having problems with breaking tank O-rings , make sure you are pointing the tank up while it is being unscrewed. Most people point the tank down while unscrewing, which releases liquid and causes hell on you tank O-rings.