I find that Killington is kinda wide and while it has 1 really long trail, for freeriding it is not the best around. The terrain is a little more spread out than I would like. My pick would by Jay if you had a choice.
My REAL vote is for Sugarloaf... Greatest acreage and steepest terrain east of the Rockies.
+2 on the 'Loaf here, hoping to trek my way up there for 5 days or so this year, but Killy can be good if you can beat the crowds.
There is NO COMPARISON. Jay is known for its trees. Killington is known for whiny New Yorkers. Jay Peak will challenge you at any skill level, and when the sun goes down and the lifts close, you can go surfing indoors.
Sadly, not much open yet, even at Jay. We just spent the weekend at Stowe, and there is no natural snow yet. We have a weekend booked at Jay at the end of January. Should be good snow by then.