Merlin seem to be doing an XT Brake package with wheels for the same price as SLX, and its a better wheelset. I know the SLX have had good reviews but is XT still better?
Better/lighter rims with the SLX set - 717s as opposed to 317s. XT756 hubs are six-bolt, whereas the LX will be centrelock/splined I just got some wheels with XT756 hubs - they're good enough hubs but will be heavier than the LX hubs. Also, the wheels with SLX use double butted spokes. I don't think it is a straightforward choice between the wheels. I think the XT brakes should be better though - i'm sure someone will post about that.
I would stick with the XTs providing they are the M775s. The power is very controllable. The free stroke & lever reach adjustment is a nice little touch. People have said they didn't really notice a difference with them, but the free stroke certainly helped me get mine set up better. I run 180mm(f) & 160mm(r). They hardly ever squeak if they do start, after a couple of wheel rotations the ServoWave adjusts the pads & it goes quiet again.