50i owner here - we bought the 50i as we wanted the 3rd row, which obviously isn't available on the X5M.
Otherwise, X5M for sure. But only a 2013 w/ LED headlights
The ///M cars are such a step up from the non ///M variants - I learned this quickly after buying an M3 a few years back. Especially if buying used, the ///M models are priced fairly reasonably.