If you need advice for the game, I suggest you read this:
http://forums.tripwireinteractive.com/showthread.php?t=44262
It's had a lot of additions by the rest of the community and I have to thank each and every one of you who's posted in there and had something that is in the OP!
Anyway, here comes the details of the weapons:
Bullpup
My personal favourite. A level 6 holds 50 rounds a mag with 10 mags total. That's 500 rounds. It has a fast fire rate, it's quite accurate and it has almost zero recoil. It's damage is quite solid as well, giving you the advantage of instantly decapping every Gorefast, Crawler, Clot, Bloat and Stalker in the area. It's cheap and can be found in the map. It's effective with full-auto even in longer ranges (though not as much as the SCAR's semi-auto).
SCAR
General rule is to treat it as a sniping weapon. It has a higher firerate than the AK-47, but it's not as fast as the bullpup (As far as I know). It's a very powerful weapon and it's extremely accurate. If you get a ZED's head in that red dot sight, that ZED is dead with one click. It does heavy damage and it could be used as a "panic" weapon for when you get charged at by a Fleshie or Scrake.
AK-47
It has 37 rounds for a level 6 and it deals good damage in between the SCAR and the 'pup. It has the slowest fire rate out of all the commando weapons but it does solid damage to more "absorbent" targets, like the gorefast's chest area. Most say its sights are horrible, while some say you don't even need to use them - this justifies the fact that the AK-47 is a close-quarter "spam" weapon. However, it's possible to snipe with it, but it is nowhere
near as effective as the SCAR.
If you're going to have two commando weapons, take a Bullpup and a SCAR.
If you're going to take only one, take the AK-47 because it's spread 50:50 between both jobs but it will never outshine the SCAR and 'Pup when it comes to actually doing those jobs.