The main thing is the barrel. The pencil Barrel on the colt gets hot fast... Not as accurate and the weight reduction is negligible. I also like the railed gas block on the smith so I can use different scopes and sights with out a tower sight being in the way.
I own one of these and love mine but I also own a Smith & Wesson M&P 15 OR and it's a much nicer gun. You can pick the Smith up from Dana Safety Supply for $750 plus tax.