i love my z560's, it's so powerful... and it's so nice... the altecs subwoofer is actually bigger, so i can't fit that in my room
that pretty much decided it to me since they were both 4.1s and roughly the same price
THX probably is a marketing scheme, but it looks nice on your speakers and you can show off to people that don't know any better...
logitech is more for the gamer and altecs are more for the music listner, so it kinda depends who you are...
i don't think you can go wrong either way... good luck deciding...