I've ridden several models of the SL (narrow version of the Legacy) and it is a solid all-mountain freestyle board. From carving groomers
, to jumps
, to jibbing on boxes and rail
I've never ridden the YES board, but I've ridden several boards with magnatraction and I don't like them... it bites too early and too hard, and makes a really rough carve (slowing you down). YES apparently has "Tragna Maction" which is clearly a copy of it (I am assuming they are licensing the technology for Lib/Gnu/Mervin). Also, I'm not sold on the recent return of the Asym sidecut tech... that was popular back in the early-'90s and Mervin is trying to make it fashionable again like Bellbottoms... I'm not convinced.
That being said, two of my friends have Gnu Park Pickles and they like them.