Deore 596 have new levers (sans knob) and old 595 calipers.
SLX 666 and 675 have new levers (with knob) and have new calipers that can take finned pads.
Shimano pads for deore are more expensive than non finned pads for slx. Non shimano pads (like superstar) cost the same for both.
I think the weight difference alone its not enough to care.
The same goes for the reach adjustment knob – one less thing to break.
I took the SLX for my latest build mainly to be more future proof, but still use the old reliable deore 595 on my singlespeed.