Not very exciting I know, but I've been wanting such a cassette for ages and noticed these two... I've been trying to run a mix-and-match XT/Ultegra set-up to get 11-27 which has never really worked as it tends to skip about around the 3rd/4th cog unless perfectly clean and aligned.
So the question is which of these is the daddy? I tried a SRAM cassette once before and it was cheaply-made rubbish, all plastic spacers and no mud clearance (and not cheap either), so have been on XT ever since.
I was about to scoff that they didn't do a 9spd 11-28. But they do!! Go for the slx it'll work fine with xt stuff.
I can't remember the last Shimano cassette I brought and I run 3 bikes and ride every day. Sram for me but NOT the cheapest one - probably the same with everything you buy, shirley !