If you like one, you'll like the other.
The lifts at jay are a bit faster.
The weather at Mad River Glen is a LOT nicer in midwinter.
Jay has one or two short pitches that are steeper than MRG.
MRG is steeper on average.
MRG has less ice.
Jay often has better cover.
MRG is $5 less per day (last seasons price)
Jay is in the middle of nowhere.
MRG can get crowded on weekends.
Jay has a tram that's toasty and and warm but is on wind hold half the time.
MRG has the single chair which almost always runs but is cold and there's no one to share warmth with.
They both have amazing bumps and glades.
Jay allows boards, MRG does not.
Both have good backcountry, but it's easier to get back to the resort at Jay.
In the woods at MRG watch out for sudden cliffs.
In the woods at Jay watch out for Large predatory Cats that will eat anything if it's a particularly cold winter, including a skier that has fallen in a deep snow drift and can't free himself quickly. Somebody got mauled last year.
Jay speaks french.
MRG speaks Bostonian.
Both are equally unintelligible.
They're both awesome.