As far as I can tell from my experimentation there’s no difference other than the skills that are activated by what you equip and, of ccourse, the trait.
Both Companions (so far) are capable of tanking, healing, and dealing damage. Armour and weapons seem to be simply a means to open up a specific skill or skill line relative to the task you want them to perform.
To be honest, I like it this way. Not too much faffing about and very easy respecs.
kringled_1 wrote: »The different destruction staves have different effects on the first skill in the tree - inferno is a knockback, lightning is aoe damage, I forget what ice does. It's the skill that's analogous to destructive clench.