The SCAR-L is better for single-shot sniping in some scenarios, because it has a much longer maximum range (1000 over the 500 of most carbines). Other than that, though, it's less accurate,
especially after the first shot. Check out stat comparisons here:
Weapon ComparisonsI gave my votes to the G36E and the P90. I unlocked the SCAR-L, and I find it to be the most worthless piece of crap around. I know some people like it, but I hate its recoil problems, and it sounds like an airsoft gun. The F2000 is good, I use it when playing SAS, but if you're happy with using the M16, then it doesn't add much variety to the Assault kit (although it does have damage 34, so you can get one-shot-kills with headshots. It's less accurate than the M16, and less powerful than any of the other assault rifles. Definitely not a first-string choice, in my opinion.
The G36E is an excellent weapon, very versatile, very accurate at all sorts of ranges. Not much can be said against it. Loses out in very close combat to the more powerful, full-auto AK's, but otherwise it's amazing. The P90 is not a bad weapon at all, really. Mechanically superior in pretty much everyway that matters to the other SMGs (except in draw time and maybe reload, I think). It means that you can add variety to your Anti-Tanking kit without having to suck up using the seriously sub-par stock SMGs. With 900 rpm and a 50 round magazine, you can tear people up in close combat, and not run out of ammo after one guy. In theory, you can also use it for single-shotting people at range, but honestly you're better off using your pistol or SRAW/Eryx for that.