Being an absolute rookie on these snowbikes, I do have a comment to make on this topic. After only two rides on bikes with a Mountain Horse kits, one with the ST and one with the LT, here are my perceptions.
During our awesome demo ride with Brett from Timbersled, I rode a 501 Husaberg with a 2014 ST kit with it's 121" track. The bike was nimble and fun but, I found myself either stuck or having to turn out and take less aggressive lines than I preferred more often than I would have liked.
Yesterday, I finally got a chance to ride my 2010 KTM 530 (isn't it a 501 cc as well?) with the 136" track on it yesterday. The bike did feel a bit longer on the trail but, I was amazed how it hooked up and just kept climbing. I was taking lines that were much more aggressive than I was able to take with the ST and just when I though I would have to turn out, that track would hook up and I would continue climbing. It may have been that the snow was more setup this ride but, I think I just experienced what others are talking about when they say they prefer the LT to the ST.
Right now, I have no intention of pulling off the 136" and putting on the 121". The 136" just worked too well for me to even think about doing that. However, the 121" is 1 7/8" and the 136" is a 2". That may have made some difference as well.