If I was gonna sell the SL, I wouldn't go Evo, I'd go Proto.
IMO, the Proto pretty much makes the SL obsolete. I'm not sure why anyone would go SL > Proto if buying new. NS is wise to keep it around as it's been their most popular board, but from an objective standpoint, there's really no reason to pick it over the Proto.
I've never ridden the Proto but I went Legacy over the Proto CTX because I skew more free ride than park or hardcore freestyle. I like speed and I've gotten into kickers, the extra dampening of the Legacy/SL over the Proto seemed right for me.
I do wish that the Legacy was a true twin though. I don't get a lot of powder in my area.