I own both a NF 8-32x NXS and a Leupold 6.5-20LR (not M4). If weight and price are not an issue, get the NF. If weight and price are an issue at all, get a 6.5-20LR and send it in for M1 turrets (well the elevation any way). Can't go wrong with a used one of either!
If you are realistic about expectations you won't be disappointed with either. Leupold may not be the "best" but I don't think more long range game will ever be killed with NF than has already been killed with Leupolds!