i was going to pick one of these up.. wanted one for years. Finally ready to pull the trigger after selling all my other junk, and now the family is just to big for a 3 series.. So i bought a 535xi.
I think the rwd would be more fun, but I have no regrets. I am actually debating buying a second one for the wife..
The 535 is such a steal for the money. Slap a JB4 on it, maybe some meth and call it a day. The RWD is a lot of fun but for a family, you probably want something that's a little less prone to get sideways. With traction control off, it takes little throttle to break the ass out on my 550.