I bought a set of the original SLXs 4 years ago and wished I’d gone for XTs. There was no difference in power, but the XTs (M775) were finished better, more adjustment and you didn’t need pliers to change pads (This really irritated me with the SLXs). All minor points but I reckon it’s worth spending the extra for that bit of extra class.
I haven’t properly compared the new SLX vs new XT but they both seem like a great deal via Rose. I decided to spend the extra and go for XTs. Mind you, it has to be said that although the new M875 XTs look bling, they do feel a bit plasticky… Hopefully they’ll last another 4 years and it’ll be money well spent.