[Silous]

I need to by a new rear derailleur. Ill be buying the DeoreXt at Jenson for $34. I'm not sure if I need medium or long cage. Is there a significant difference?

[Dstompe]

if you run three gears up front and 8 or nine in the rear go long cage, if you have 2 un front and 8 or nine in the rear go medium cage

[tajcrews]

I would go medium cage. There is some slight shifting difference but not a lot. Medium also does not hang as low on your frame so less likely to get catch a stick or something too.

[WellnessCoachGreg]

Need more input. I am assuming that you have a tripple chain ring front. If you do, and it isn't to old your gears should be 22,32,44. The important part about this equation is your cogset, if your big cog is 32 teeth or greater, go with the long cage. If not go with the medium cage. Make sure to attach the chain while in the small chain ring to the small cog, and allow for as little pull on hte rear derailer as possible. That should creat for better shifting.