I suspect you are choosing between two fantastic SUVs. I also guess that, comparably equiped, the X5 V8 will cost between $5-10k more. But if I were choosing, I'd still get the X5.\
BMW's are NOT trouble-free, but IMHO they are just extending their lead over Mercedes and everyone else as creators of the best driving (if not always the best looking!) luxury vehicles.
If you may, treat yourself, get all the goodies on whichever one you choose, and enjoy the fact that your vehicle costs 30% less to buy than it would in its native Germany (including our value added tax over here... now 19% in 2007!) and less than half as much to run!