You didn't mention what type of riding you do. Leo makes a good point: when you go from 157 to 161, you change from the "blunt" shape to the "pointy." I ride the pointy version (161 and i'm 5'10" 180 lbs.) and haven't ridden the blunt shape so can't directly compare. i can't imagine you need to go bigger than 161.
I doubt the difference is extreme, but the blunt reportedly will have more of freestyle/park feel (though still not real soft) while the pointy is intended to be the powder/freeride version of this all-mt. board.