Colt 6920: $1000 (from Walmart...unless you're in CA)
Aimpoint PRO: $400 (probably cheaper if you look hard enough)
For $1400, you'll have the same thing as a M4 (plus 1.5 inches). Do it!
Good call. I honestly haven't looked into it in a few years.
Frankly, for home defense, the Aimpoint is totally unnecessary - it does look awesome though.