I would think twice about an M5 for long trips because it has no spare tire. The can of goop substitute for a spare will result in a very real inconvenience. With flat on a M5 you can either (a) maybe plug it and limp somewhere below 50MPH to get a new tire or (b) deal with roadside assistance. In both cases, you will likely end up stranded somewhere for a long period of time and end up with a tire not the same as the others. With a 540, you mount the spare and keep going.