SRAM recommend the long cage version if you want to use a 44/33/22 crankset and 11-34 cassette. but this of course is only if you 'accidentally' (or otherwise) shift into a 22-11 gear ratio (and need the derailleur to take up the slack) or 44-34 ratio (where you need the chain length to stretch this far). given these are extremes, and not your usual riding-ratios, you can get away with a medium cage if you take care to avoid them.
the situation is slightly worse with FS, long travel, bikes. as you compress the rear end, it will stretch the drivetrain distance a couple links worth.
have a look at the SRAM tech manual:
http://www.sram.com/_media/pdf/sram/dealers/TM_MTB_MY06_E.pdf
personally, i'd go the longcage i your situation. the benefits (lighter weight, more ground clearance, possibly snappier shifting) of the medium cage - in general - probably don't outweigh the possible problems.
