the YHM cans are absolute beasts; warrantied for automatic fire on short barrels. i've beat the crap out of mine, and it still looks and sounds like it did on day-1, despite spending all of its' time on my little 10.5" guns that i use for class. they are functional, relatively small, quiet, *very* overbuilt...
if it were between an M4 1000 and a YHM, i'd tell you to run with the YHM. but the M4 2k is a better can. you'll maintain the same sound, but in a slightly smaller and prettier package. also, the biggest downfall of the YHM is its' weight, which is one of the biggest selling points of the M4 2K; it is a significantly lighter can IIRC.
you are also looking at two very different price points though, unless you are getting ripped off on the YHM or are getting a SMOKING deal on the M4 2K.