I have both. There's not a lot of difference between them. On the lever end, the XT has the free stroke adjustment, and the lever itself feels like it's made of solid metal (probably hollow though), while the SLX lever is made of bent sheet and feels a little cheaper. On the caliper end, the XT is a monoblock design while the SLX is two-piece. The pads on the XT are secured with an easy-to-use threaded rod, while the SLX uses a cotter pin.
Other than that, not much difference. Weight-wise they're almost the same (my SLX set was actually lighter but it might have been due to different length hoses). I like the XT better, but only by a hair. I'd go for SLX if the price difference was significant.