Ive skied both, but killington was early season.
Off trail terrain wise, these two are pretty close. Tremblant has sick tree and bump runs and some really steep stuff. Killington does to, but ive never skied it. Tremblant's parks suck though, and killingtions are sick.
I would definitely go with Tremblant because it is waaaay nicer in terms of lodging and the lifts are really fast. The lay out of the mountain is really cool. Both are sick mountains so you wont be disappointed either way.