Mullet hunter. They are about the same, the colt m4 is 6lb 11oz. The sig sauer m400 enhanced is 6lb 7oz. In fact the stock m400 is 6.5lb, lighter than most ar rifles. Yes that is empty weight. 516 is a little heavier 7.6lb or somthing like that.
The Sig AR comes with a nice black padded carry case with the Sig logo. A lot of shops, even Wal mart will not include the case with the gun, so make sure you get the case. I saw Jay's had removed the case from one and was selling it on the floor for over a hundred bucks. Also, a buddy of mine bought one from wal-mart, and demanded the case they said they didn't have, but a manager found it in the back, Hmmmm.
I have had several sig 556's and they have never had any failures over thousands of rounds of the cheapest ammo that I can get my hands on. Great guns, and the prices have come down a TON. I wouldn't hesitate to pick up one for 1k or less, just steer clear of the models with the collapsible folding stock, its junk.