Just my .02, I wouldn't lease a loaner, or buy a loaner. They are beat on by everyone.
I've spotted people smoking in loaners. God knows what else they do in loaners. With BMW's big discount drive on now, I would think that the price diff in new vs. used is very small. Buy new with some peace of mind.
I don't beat loaners, but a lot of people do. Remember you're still paying a good buck plus tax and interest for someone else's beater. And the very good question comes up....what if you actually want to buy this car at the end?