Just to start, i'm 5'10 170lbs, got the 158 '09 SLR last year.
It really depends on your riding style for freeride, i think the 158 is perfect. stable, fast and floats in powder when you need it.
In hind sight, i probably would have gotten the evo-r 155, but thats just because i've moved more into freestyle from freeride. i really want a twin now.
anyways either size is fine. for me, i think the 155 is the best, you can do butters and small spins. makes the board a little more fun.