M&P Sport, sell the upper and buy something else with the $400-500 extra. I assume it can be used on multiple S&W firearms? Maybe an autoloader or a wheelgun to finish out the balance.
No sense in buying a premium S&W if you are getting rid of the upper anyways, the lowers are all going to be very similar between all their models, minus some minor changes that can be rectified for low cost and ease in the future if need be.
Personally, I think every gun owner and especially those who own AR's should own a 5.56 upper. Very affordable to shoot without reloading for it.