Re:
I have the previos version of the Deore brakes, M590 on two of my bikes and they're absolutely fine, dead easy to set up and the finish has lasted well; they still look pretty much new. I haven't tried the XT version, but there isn't much to a V-brake arm, so it's difficult to see how they could be improved.
Some versions of Shimano V-brake arms had a parallelogram linkage arangement. I haven't tried them, but I have read some complaints about the linkages getting worn and rattling; so I'd probably avoid those.
The XT levers are different, one of my bikes has Deore M590 levers, the other has XT M740 levers. The XT levers have a slotted hole for the cable attachment, which decreases the cable pull as the lever is pulled; I think it's supposed to increase power and improve modulation. However, the effect is pretty subtle and switching from one bike to the other, I can't say I really notice much difference.