Long Cage XT, with explanation
The biggest thing that determines whether or not you need a long or short cage derailleur is your cogset. Assuming you're running 3 chainrings in front (you don't say) and an 11-32or 11-34 cogset (you don't say) then you would want to go with the long cage, so as best to take up all the chain slack as possible. if you have a smaller cogset, only run 2 chainrings, or are super careful about not being cross-chained, then the mid cage would be ok. But otherwise just get the long cage.
As far as the Saint derailleur goes, you need the Saint hub to match it; instead of attaching to the frame like most derailleurs, the Saint attaches to the hub axle. So unless you have a hub with a compatible axle - Saint or Hone (I believe they're compatible) - you can't use the Saint derailleur. The Saint derailleur is currently only available as a low-normal (RapidRise) but supposedly there will be high-normal versions coming. So unless you've got the new style dual control levers, the low-normal derailleur isn't going to shift as nice.
So basically if you want the Saint derailleur you need a new rear wheel and likely new shifters as well.
Get the XT. When you bang the hell out of it, it will be cheaper to get a new one. I've replaced my XTR stuff with XT as it wears out, the XT stuff is 90% as good and 50% as expensive.
The biggest thing that determines whether or not you need a long or short cage derailleur is your cogset. Assuming you're running 3 chainrings in front (you don't say) and an 11-32or 11-34 cogset (you don't say) then you would want to go with the long cage, so as best to take up all the chain slack as possible. if you have a smaller cogset, only run 2 chainrings, or are super careful about not being cross-chained, then the mid cage would be ok. But otherwise just get the long cage.
As far as the Saint derailleur goes, you need the Saint hub to match it; instead of attaching to the frame like most derailleurs, the Saint attaches to the hub axle. So unless you have a hub with a compatible axle - Saint or Hone (I believe they're compatible) - you can't use the Saint derailleur. The Saint derailleur is currently only available as a low-normal (RapidRise) but supposedly there will be high-normal versions coming. So unless you've got the new style dual control levers, the low-normal derailleur isn't going to shift as nice.
So basically if you want the Saint derailleur you need a new rear wheel and likely new shifters as well.
Get the XT. When you bang the hell out of it, it will be cheaper to get a new one. I've replaced my XTR stuff with XT as it wears out, the XT stuff is 90% as good and 50% as expensive.
