Might as well get the extended mags since I'm guessing you've already unlocked all the other attachments. Easy XP
(Unlocked my SCAR EM a while back, and just unlocked FAMAS EM last night).
The only, only, gun I've regularly used extended mags for is the AA-12. For the rest, there are better/more useful attachments.
Unlocked EM is difficult though, you will wind up only getting 3-8 a round, especially if the rounds are short or there aren't many people. It also depends on what map you play. Terminal stick out in my mind. Be sure to use Bling and attach an HB sensor as well; locate a red dot, guess where he might be hiding, and fire away.