Holy crap I can't remember the last socom related poll I voted in.
I'm a huge fan of the M4/552 with the laser and grip combo. Used together I can't believe it hasn't been ripped apart as being a noob combo. If you're a runner and gunner and haven't tried the combo, you're missing out.
As for which is better, it depends what you want out of an attachment. The laser keeps the bullet spread minimal on full auto. The grip keeps the bullet spread minimal when running around.
So if you like to run around and have optimum accuracy while shooting everyone in sight, try them together. Of course the only real drawback is you need decent aim. ;)