Ever try to get through 3'+ of powder with a 136? Ever get to the big hills later in the day, and all the good runs have been trenched down so far you can't go anywhere? Its no problem for the guys who have 159's. I have a 151x2, and that gets me anywhere I have the balls to take my sled and most places I don't, but every weekend I watch the really long track sleds go into mountain bowls I didn't think were possible to touch with a sled. On the other hand, watching these same guys try to negotiate a tight trail is mildly entertaining, its not easy to turn a track that big, and once they're stuck it takes 5 guys to pull their sleds out. I honestly think its more about big lugs than it is tracks, at least for climbing. I also have an indy 550 fan w/ a 136x1.5, and I'm amazed where it can go (only weighs about 450 lbs dry). Whatever you prefer I guess.