I chose stone before duress, becuase stone has some massive defence (around 3k in a eth armor). It also has a good fhr wich is important for a act2 merc.
I'm playing a orb/charged bolt sorceress, and I use my merc as a second tank. There's pretty much no monster in the game that I cant kill myself, so the merc doesn't need to be that offensive.
Also, the "lack" on resist on stone isn't really a problem (atleast not for me). I hired my merc in normal and me and my merc is lvl 93, so he has maxres in hell (with stone as the only item that gives him resist).
However, if you're a single-tree sorc you might want to consider duress for more killing power.
