Orphan said:The only thing you really need to remember is that a merc hired on a lower difficulty will have better stats than a merc hired on a higher difficulty. In other words, a rogue hired on normal will have better strength, skills, dex and HP than a rogue hired on Hell.
For Act 2 Mercs on NM, you won't have much of a choice. Apart from that, nothing else matters.
FrostBurn said:Hire the highest. If you hire the lowest, and level him to the highest, ALL his stats and aura will be identical to the one who was the highest i.e. there is no difference apart from the merc level on the hire screen.