I just picked up a NS 2010 Evo, and boy is this thing WAY different then what I have,(Atomix Hatchet). Plenty of flex, I strapped mine in on some carpet and was throwing the thing around like a tennis ball. I can't wait to butter this thing in some powder, the nose is going to go so high! While it does have lots of flex, it's still not noodle-esque though so I think it will handle much higher speeds then the average park board, it may also be a bit more stable landing jumps since the thing doesn't go "Boing!". The Rocker and sidecut designs also help making it a pretty catch free ride, something that is nice when your landing tricks. I understand it handles pow pretty well also. Kinda of a jack-of-all trades while still being a solid park board. If you do go with the Evo you may want to stay 151, just because the way these things ride you can go a little bit bigger then you normally would.
From what it looks like the Believer is more of a kicker/freeride/freestyle board. If you plan on more of a freeride experience with the kickers thrown in, this may be a better choice. I think though if your planning on hitting jib features and preforming flatland tricks, along with everything else, the Evo will be better.
PowderHound and TreeNinja