We had this debate a little while ago if you do a search, there where a lot of responses.
I own a Dissipator, and I have shot shorties quite a bit. It is really a question of personal preference. The Diss has a longer sight radius, I think that it helps me to shoot better, if you have optics then it is a moot point. I am a big guy, and I like having more of a handguard to grab. It does feel more muzzle heavy than a shorty, but once again that is a matter of personal preference. Some people think it is ugly, I think it looks better than a shorty, just my opinion. (it does get lots of looks at the range) If you are looking to mount a tac light, there seems to be a lot more options availible for the shorty.
It all comes down to opinion, when I decided to buy an AR I went out and looked at every variation that I could get my hands on, and I finally decided on the Dissipator, and it has never dissipointed me. (1 inch groups from a bench rest with hawksview ammo) My friend wound up buying a shorty with A1 sights, because it is what he liked the best, and another friend bought a flattop Armalite because he liked the green stock. They all work equally as well.