The SRAM X-7 and X-9 derailleurs are both excellent choices. Personally I would run with the X-9. Here are a couple things to remember when selecting a rear derailleur.
Long-cage derailleurs have more distance between the pulleys than medium-cage and short cage deraileurs, allowing them to take up more chain slack. If you only have one or two chainrings, you can probably use the medium-cage, but if you have three chainrings and the usual wide-range cassette, then long-cage will be more suitable. The longer cage is slightly heavier and rides a little closer to the ground, reducing obstacle clearance a little bit. RapidRise derailleurs are sprung so that they "home" on (return to) the largest cog on the cassette, not the smallest. I think the theory behind RapidRise was that you could force-feed upshifts to smaller cogs instead of relying on your derailleur’s parallelogram spring to upshift when you let out a click’s worth of cable at the shift lever. RapidRise or non-RapidRise is your personal preference, just make sure you have shifters compatible with a rapid rise derailleur (STI style). Furthermore, the manufacturer publishes a "Total Capacity" for a derailleur in the specs which can be found in the owners manual or most likely, via the internet . The capacity is calculated by subtracting the smallest chain ring from the largest plus the difference between the smallest cog on the cassette from the largest. On a standard 22/32/44T crankset with an 11/34T cassette The calculation is as follows:
(44-22) + (34-11) = 45
So with this combination you require a 45T capacity derailleur. Most medium cage derailleurs have 33T capacity. If you rings and cassette work out to more than 33 you need a long cage. Hope this helps you out somewhat 😄 .
here are a couple links to jensonusa for derailleurs as well. Hopefully this info will do you some justice 😃