I’d be quite happy with SLX, but the price difference aftermarket at CRC/Merlin/German sites is so small (£70 on a full brake groupset), it makes XT a no-brainer personally.
XT also has a cassette that’s easier to add a range-extender to.
SLX rear mech looks better though 😉
The only bit of XT I find disappointing these days is the shifter. I used to love their 9-speed ones (especially Rapid Rise).
So I’m looking at either Saint or XTR for that bit.
There’s also a difference in feel between the SLX/XT brakes and the Zee, so that comes down to weight vs modulation.