One thing I might add to this thread is that it really matters who your playing against. If your in a session with a consistent group you will learn who is doing better against you than others, along with what gear and tactics/style they use. If someone kills you ten times with an auto, then picks you off with an sr8 out of nowhere, pay attention to the fact that he changed up on you and adjust.
Personally it works well for me to counter what particular gear/style I am up against. If I'm fighting a skilled sniper I'll charge/flank/kill with an auto, if I'm losing the auto spam fights for whatever reason I may change up to a scoped weapon. If I'm on a bloody arse tearing rampage I'll switch to a baretta, and start the run with the pistol until I use up the ammo, then switch to a secondary auto, then a primary auto in order to maximize the lethality of my run.
There are really a lot of factors to consider and several different ways of looking at those factors. Environmental factors make a big difference. Pick a home server with a group of regulars you can get on well with and learn the map cycle. Make tactical adjustments based on an order of priority, like first what sort of map is it, then how many people are in-game, then how is my lag at the moment, then who is doing better/worse than you and why, etc etc yadda yadda you get the point.
As always, make fun your priority.