SLX come with removable shifter windows and blanking plates
I believe the difference (I’ve ‘stood-tested’ both, though at different times) is that XT (2008-2011 9spd) changes slightly quicker
On the release lever (ie going to lesser-toothed-sprockets/harder gears on ‘normal’ release mechs (not rapid rise), I think XT is supposed to release as soon as you push the lever, whereas SLX won’t release until you start to let go of the lever? In reality so long as you don’t have some sort of habit where you hold onto the release lever for too long, its going to be a very small difference
I could of course be wrong in thinking all of this ..
“Unless you want 2 way release”
Which Deore 9speed has these days (at least from around mid-2008), though I imagine non-2-way release deore shifters will be hanging around somewhere online and probably came supplied on a few bikes in the meantime?