Occam's Razor: All else being equal, the simplest solution is the correct one.

You really aren't going to notice much difference, if any at all, between XTR and Deore shifting. If you ride the bike in all weather, the nicer finish of the higher end parts won't matter. Some parts have to be replaced regularly and so it makes sense to use the parts that have the best durability to cost ratio.

I haven't used XTR. I have used Deore and Deore XT. I think the M590 Deore components shift as well as XT. It's quality stuff.