I was in a similar dillema and went with the Ascends for their slightly lower extension, but the X-LSs were very tempting at that price point!
My suggestion would be to put a pair of 170s on order, side by side them, and then send back the loser. You get the satisfaction of knowing you made the right decision (kills the second guessing a year from now), and at most you're out a few hours of enjoyable auditioning and return shipping.
~Josh