In my opinion, both of them are the best of both worlds; one is better at one thing and the other is better at what the other does not.
I do believe that ES will tank better than a Vitality Sorceress (although both can definately tank a fair number of good hits) if that's what you were getting at.
Vitality, however, has the ability to pick up a second skill (best from the Fire tree) for kicks (the left side of the Fire Tree) or for support (right side). Inferno/Blaze/Fire Wall can make for some entertainment, but for more serious PvP, Enchant can help a lot in team duels while Hydra, when used right, can help "guide" your opponents into a general if not specfic direction.
You could look at it like an ES Sorc is a pure "solo" dueler while a Vitality (in Fire's case at least) as a more party friendly one, yet just as capable of solo dueling.
Also, not that you asked, but in terms of 1v1 Vita vs ES, it'll almost always become who gets in their attacks first.
EDIT: And if it comes up, Lightning Vita vs Lightning ES will boil down to "do you want more tanking or more damage". And for Blizzard's case, it'll end up being which one you like more, Vita or ES.
This post was edited by ShugoTenshiKrys on Aug 27 2006 10:10am