Short tracks are usually lighter, so will take less HP to go the same speed. They usually corner better than longer tracks. A medium track will usually be better on the stutter bumps, and often will out-accelerate a short track because the track hooks up better. Longer tracks offer better flotation for those days of killer powder or if you want to get off the trail.
Welcome to the forum!
2007 Dragon RMK 700 155", 2008 RMK 600 155", 95 Prowler 550 2-up, Really OLD AC Cheetah
Welcome to the snow show. IMHO a 136 is perfect for trails. A 121 is more maneuverable but doesn't take the bumps as well. Anything longer than 136 is not very maneuverable, and the track is usually a deep lug track which will cause cooling problems.
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.