I would recommend a High Elf/Altmer over a Breton, mainly because their magicka refills faster. They're not too bad with one-handed weapons, either. When I played as a Breton during a practice run, I found that although they have more resistance against other magic wielders, their magicka refills rather slowly. A successful battlemage utilizes destruction and restoration magic in addition to staves or one-handed weapons, and the faster regeneration helps. I personally play as a High Elf battlemage as my primary character, and I'm satisfied with the result.
Plus, High Elves start out with extra magicka.