What's the difference (other than the price) and do you notice it?
Non that’s discernable. XTR on the 5-spot XT on the on-one, both just work as did the 595’s before.
There's hardly any difference between XT and 540s either, and not much more between 540s and 520s. (That said, 505s are crap) I wouldn't go any higer than 540s myself.
Longevity would be my only guess. I have a set of 540s and a set of XTR which were bought at the same time and used very evenly. the XTRs still spin like new with no play in them, whilst the 540s will probably be retiring pretty soon. Other than that, couldnt tell the difference between them in a blind test.
M540 = XT in different colours.
XTR lighter axle and coated bindings are the only difference im aware of.
M540s for FTW.