I do not like the Westin Cancun as much as the Hilton, as it is located near the club Med and rooms arenīt as beautiful as the Hilton rooms. but, I have always been upgraded at the Hilton Cancun to the Villas, so I am biased.
If you want a Westin at CUN, pick the Lagunamar, as I always have wanted to stay there, but I am kind of pro Hilton because of the Gold VIP status I held.