That is all stuff that YOU need to figure out. Everyone plays different, and until you find the style that you are confterble with, you may have to put off getting nukes. I am a fast rusher, so I play on a slightly higher sensitivity, with a silenced UMP, the usual M,SP,N/SA perk setup, or a SoH,L,N setup. I get on top of the other team, kill them with speed, and then let the KS carry me when I have no ammo to a nuke. The problem is, 90% of people cant pull off rushing like I can.
The point is, find a class you are conformable with, and a style that you do well in, and use it. Not what other people use.