I've done a ton of research on this subject but have not been able to get one yet. My vote is for the M&P 15 with just the flat top so you can add your own sights. A lot of people like the Sportical but it lacks a forward assist and a dust cover. This is not a huge deal if you don't expect to really punish the gun and to be able to always keep it cleaned and lubed. However, in a long-term defense or SHTF scenario, that dust cover and forward assist will be what keeps the rifle running. AR styles get a bad rep as being slightly unreliable but that is if they are not taken care of slightly. Those two components will make it incredibly hard to keep clean in one of the two scenarios.
The bushmaster has the forward assist ant the dust cover for the same price as the s&w is it not a good one?