If your M5 is a 6spd, I wouldn't trade. Personally I would keep that m5 for as long as I could, as much as I love my X6M, I miss the foot clutch.

You will not be unhappy with the X5/6M. I love mine. Plenty of power, nice refinement, plenty of room to haul junk around (more in the x5m of course). I have no kids, so I can get away with it.

The only gripe i have is limited mods. It's hard to find simple things like lips and rear diffusers for decent prices, they're quite inflated over the m3/m5 cars. And less to choose from.

I don't regret my X6M, but I do wish it was manual.