Dark Elves have better DPS than Bretons. Their boosts to elemental damage (especially fire) outweigh the extra 1% Magicka you get from playing a Breton. Between the small boost to lightning attacks, as well as the large boost to Blockade of Fire, attack weaving, and the fire side of Ilambris, Sorcerers deal pretty substantial fire damage, and Destructive Ancestry pretty heavily beefs up all of it.I don't put a lot of stock into being obsessive with the racial passives. If you want to maximize and feel you're an a+ player that needs the best of everything, sure. For me...I like having fun...if my guy has 3% less stamina but he works and I like his look, so be it.
Dark elves make really good magic nightblades. I can see an awesome magblade w a dual wield attack bar and destro fire retreat. I'd also be fine with a dunmer stamblade...it's not the end of the world.
Brentons are considered one of the top two races for mag Sorc. Altmer has slightly higher dps, where bretons get a little better sustain and resistance. But again, you can hone either with champion points to fill out any weaknesses...my mag Breton gets a little more into damage and my altmer gets a few more into resistance and honestly I don't think the race makes a big difference.