Killington is like one of those OCD things for me. I hate it and swear never to go back but then end up there again (usually because some friend would talk me into it) expecting this time I'll feel different about it. Which never happens. Killington is just not a snowboarder's mountain in my opinion. Plenty of flats and moguls, long green flat runs. The base area is an atrocity, I can't in all honesty, say I've seen a worse layout in a major resort. However, their parks are very good. I love the Stash with its cool features and they have one of the best progression parks in the East.
Tremblant is definitely superior in terms of terrain layout. Advanced Zone park is great, others - nothing speacial. There's an Alpine style party village at the base, something Killington doesn't have. The place is markedly more expensive and awash in drunk kids.