NOOOOOOOOOOOOOO!!! Don't do it. It's called seller's remorse.

Hahahaha. I'm just kidding. I realized it all boils down to what you really want. Don't you just love the x5? You feel grounded and so secure in that vehicle. I say keep the X5.
I dunno, what your needs are but have you checked out the lexus rx400H? Price is steep but it I guess it makes up everywhere else. And I have to say it's dead quiet.
Damn I wish BMW made a hybrid X5. My vision for BMW: a 400HP hybrid X5 with 50 miles to a gallon.

I may decide never to fly to any state if they come up with this. Hahahaha.