MW2: What gun should I use next?

I am a completionist so it would drive me crazy if I didn't go ahead and finish the gun. However, sometimes I get bored easily using the same weapons so I like to switch. In this case, I would stick with the UMP just to finish it and get the bonus xp for completing the challenge, then I would move on to the Scar. It's actually a really good gun once you get the hang of it. I had friends that didn't like it, but it has good accuracy and is an easier gun to get the bonus xp fast.