If you’re thinking of putting an extender cog on then I’ve heard that the zee mech struggles to cope but doesn’t meant it can’t do it with a bit of fiddling. I’m not bothering going with an extender myself, I’d rather stand and mash the climbs out than spin away.
I’m 1×10 with an SLX mech, NW race face chainring and I’m yet to drop a chain. I’ve ridden all of Afan, local natural stuff and an uplift day at BPW so my vote is SLX.
They both work fine but the cage lengths are very different. Zee works great with an 11-36 cassette on a hardtail. Some full-sus bikes (like my Spitfire) have too much chain growth for it, so medium cage SLX works much better for them. With an expander cog on any bike I wouldn’t even consider Zee.