I think the Evo is just a little stiffer than the SL, not much though. I think NS rates the SL a 5 on stiffness and the Evo a 6 (1 being the softest; 10 being the stiffest). They both have incredible pop though. You won't be disappointed w/ either of these boards.

Yeah, the NS SL 158 sounds like it will be good for you. It's PERFECT for every where you want to go. I ride deep Utah pow and the park with no probs. Built like a tank (you'll notice this on your landings). I'm only 5'9", 155 lbs, so I don't if you would want to go to the next size up or not. I would say if you want more freestyle, go 158, more freeride then go up the next size (161 I think). Let me know if you have any other questions, I'll be happy to answer whatever I can.