The are MUCH lighter, if you run it way back towards the BUIS it makes it much better in IMHO.
Of course, doing the same thing with a M4 gives you even more.
That is they way I run mine.
I like the M4 because of the clear Aimpoint Factory lense covers, the 2 screws into the housing for the mount, the easy to adjust rheostat and the AA battery. Which being an X-box player, I have lying around in copious amounts.