The Deore XT M750 has been superseded a couple of times, but there shouldn't be any incompatibilities replacing it with a later derailleur.
Any one of LX, SLX or XT would work fine.
The closest current match is the Deore XT RD-M771. You might want to consider the XT Shadow, RD-M772. This has a simpler cable run, and the derailleur is tucked a bit closer so it's less vulnerable to rocks or branches, or creative airport luggage handlers. I haven't used it myself, and I've seen some reports that the shifting is not as precise as the conventional model, but I tend to take reports like that with a grain of salt. Shifting performance depends a lot on cable run, and cable maintenance, and I suspect these would outweigh the differences in the derailleur.
Getting a mid rather than long cage model will reduce the chances of picking up another stick in your derailleur, (and reduces chain slap) but you have to be prepared to avoid cross-chaining. I'll probably go this way on my touring bike if the current derailleur ever gives up the ghost.
You can save some money by going for the SLX or LX models, but the saving is not huge, unless you get old stock, or something on special.
