Test drove the X6 50i today:
minuses:
- Less room than the X5
- Less visibility out the rear than the X5
pluses
- More planted and better cornering than the X5
- More unique (+ve or -ve is a personal opinion

)
Given that we don't have kids (and aren't planning on any), I've decided to go with the X6. It is just so much fun to drive. I love my X5, but a change will be nice, while still staying in the same family