this is from what i know/can tell, could be wrong:
Elemental resistance effects ONLY That element, but from all srouces, and it is easier to get.
Spell resitance effects magic of all kind, but not stamina/weapon related and is harder to get.
Because vampires are weak to fire specifically, Dunmer is probably the way to go if you want best defense against fire
Again, i COULD be wrong, not 100% sure.
When spell damage is applied, you can mitigate up to 50% of the damage but only the highest resistance is applied.
So if get hit by fire and your fire resistance is higher then your spell resistance, then that is used. if not then spell resistance is used.