Wizard against wizard, Breton for sure. For a simple reason, Bretons have a huge resistance to magic while High elves are the opposite, having a weakness to destruction, which a pure mage would have.
I would say the opposite.
An Altmer wizard isnt worth his salt if he doesnt counter his weaknesses with enchantment or spell.
I always play an Altmer and I know thats what I do lol.
Bretons may have bigger innate resistances but an Altmer is more intelligent and comes from a more magically inclined culture.
Therefore his grasp of magic would be bigger.
I think it would depend on who is most experienced, most well versed in the arcane.
If these are both equal and on master level, the Altmer would win.
If they were both novices, the Breton would win.
But thats just what I think