Who do you think is better for playing a mage? Also, which is more adaptable (if I want to maybe change from a pure mage to an assassin or a warrior, even if it is just to complete the dark brotherhood/thieves guild/companions questline).

High elf. The +50 mana pool is unique and priceless. For the magic resistance use Atronarch stone and get appropriate perks from Alteration tree, not to mention enchantments.

High elves start with bonuses to every mage school (and enchanting), +50 magicka and basically unlimited magicka for 60 seconds a day.

Bretons are much more useful as warriors.

I prefer breton. I find that the magic resistance against enemy spells is more valuable than the magicka bonus, particularly if combined with the Lord Stone.

Used to be Bretons and Altmer were the best mages. Now, Altmer are the best, and Bretons are the best to go up against enemy mages. That +50 magicka is useful for anything and everything, Breton bonuses are not much use against melee enemies with unenchanted weapons.

You can easily max out magic resistance with Alteration tree, appropriate stone and enchantments. No matter how much you boost your Altmer's magica with enchanting and picking skills, you will always, ALWAYS have 50 more magicka than everyone else.

also, is it possible to make a decent looking breton female? because i've tried over and over again and i cant make their faces not as round, and i cant get rid of those lines on their forehead..

Veronica begs to differ.

If you aren't going to abuse enchanting (or if you're on PC), I would choose a High Elf. That's the route I will take on my next playthrough. The armors from the Deadly Dragons mod offer quite a bit of magic resistance, plus the +50 mana and Highborn would be very useful if you aren't reducing school costs to zero.

That being said, I have always chosen Breton because magic resist is the #1 most important thing for a non-stealth character.

You can get 80%+ casting reduction with items you find in the game, no enchanting required (although it would be faster and probably easier)
Bretons with Atronach stone can be totally immune to magic at level 1. There is no other way to do this.

Right, but that is for one school. If you use summons, and flesh spells...mana runs out VERY quickly. This wouldn't be a problem if the in-combat regen didn't svck so badly.

Let me refine my position: if you are on PC and using a magic mod, I would use Altmer. Any other case, Breton. Hell, even my melee characters are Bretons.

HIgh Elves make better offensive mages (Destruction as main offense), Bretons make better defensive mages (Conjuration as main offense)

I don't think it's that cut-and-dried. Their racials certainly contribute to their optimal role, but they don't define them. The initial bonus is easy to overcome.

My Breton is a powerhouse. I walked into the chamber where Malkoran stood, launched 4 dual-cast Ice Storms, and everyone died, including Malkoran's Shade. The initial outlay of skill bonuses didn't hurt me.

I use a Breton for my mage build, but that's because 1.) aesthetically, I like Bretons more than High Elves 2.) I like the Conjuration bonus Bretons receive 3.) Bretons fit my character's roleplay better than a High Elf.

I imagine that extra magicka for High Elves would be invaluable however, especially if you focus on Destruction, which I don't.

High Elf.

+50 magicka.
The magicka regen from the Highborn power is pretty amazing in a tough fight.

Plus they just feel more mage like when playing.