I unlocked the SCAR-L and found I didn't like it at all. Not accurate enough when on auto-fire for my liking, and the damage boost is pretty negligible. While Spec-Op'ing, I stick with the G36C.
I haven't unlocked the MP-7 or the P-90, but I've used them both in SF and EF. The P-90 is pretty poor, not really an improvement on the basic sub-machine guns, but apparently it got a boost in the latest patch, so I'll have to give it another go. The MP-7 is a fun little gun. Honestly, it's about as much use at range as a shotgun, but in up close and confused quarters, it's very good at filling the air with lead. As engineer, you're probably going to need to get the drop on people to take them down one way or the other, and as long as you do, the MP-7 should shred 'em nicely.
So... I wouldn't unlock the SCAR-L and probably not the P-90. Looking at your stats page, your other locked weapon is the MG36. If you like the rest of the family (G3, G36C, G36E), then you'll like the MG36 too. Brand loyalty and all that :]