How long is it that you have have been playing? For me, I only buy gears that are going to help my game style. If you are new, your style has not set in yet so take it slow.
I would say aside from all the random stuff you can buy, pistol is a great addition to your primary. It will be helpful in tight corners or if your primary dies half way in the game.
If you want to compare gain over value, rifles are the best. But will you go in a game holding two rifles?
If you are from toronto, I go to siege once in a while. you can borrow my TM hi-CAPA for a game or two to see if you like it.