Comp M4, you ain't shootin for 1" groups with an SBR at any distance so why spend the money on a precision optic. Buy the optic for the purpose of the gun and a SBR is a sub 300 yard gun imho.
other: EOTech
it's the only one i have experience with, and it's quick and designed for up close. Like 3rdRcn said, won't be shooting much past 100 yards so why go with something magnified.
What do you guys think? I figure it would be nice to have magnification if I need it.