The folks over at LuckyGunner posted results of a test between brass-cased and steel-cased ammo in an AR. I'd say it was fairly scientific and enlightening, to say the least. They burned up something like 40,000 rounds, so I'll take that to mean they were serious about getting a decent answer.
Long story short - brass appears to be more reliable, but steel can get you by in a pinch. I was especially surprised by the lack of extractor wear between steel and brass cases.