Now that I've converted my ZR 600 from a 121 to a 136, I'd have a hard time going back to a 121 again. The trails I ride are not exactly trails. They are rough ditches with varying snow cover and sometimes lots of ruts, bumps, rocks, washouts... The 136 just rides better over that terrain.
1) 2004 Yamaha Warrior
2) 2006 Yamaha Nytro