From looking at SeatGuru (I fly VX all the time but I have never flown JetBlue) it looks like the main difference is that the economy seats on JetBlue have an extra 2 inches of pitch, which is a big difference (although pitch doesn't necessarily equate to legroom, since they can play tricks like making the seats thinner). On the other hand VX has a first class, and JetBlue does not.

So I'd probably pick JetBlue if I wanted to fly Economy, and Virgin otherwise. "Even More Space" looks like MCS without the free food and drinks.