Sorry i should have said more clearly.
The difference between an SLX and XT Cassette is that the SLK has the 3 largest cogs on a spider where the XT has the 5 largest cogs on a spider, so the XT is better but i was trying to get across that it doesnt matter.
Theres not really much difference, at the end of the day there is no cassette that will not mark a alloy freehub but it doesnt really matter. The freehub gets marked? so what? my freehub has had about 5 different cassettes on and its still going strong its just makes it a bit harder to remove the cassette. I wouldnt get to worried about it, if you think its causing a creak then put some grease on when you reinstall it.