Both carriers do route-specific promos. I personally have had better luck getting them on UA, but I suspect it's fairly anecdotal. (If I had any need to fly to Dallas in the past year, I'd give the opposite answer: MCI-DAL/DFW has had huge bonuses on AA.) UA had some nice West Coast bonuses about a year ago that I was lucky enough to hit with my regular business travel. ^

One hypothesis: you'll see better route-specific promos long-term on UA since it could feel the need to defend its hub against any number of carriers to any number of destinations. Or...simply the need to promote business on any number of underperforming routes out of SFO. It's unlikely that SFO to another airline hub city will be an "underperforming route", so you'd probably only hit AA route-specific bonuses if they decide they really need to get aggressive against UA in your market.

But this is a guess...I'm sure Bay Area fliers can provide better info on what really goes on as far as those kinds of bonuses on UA and AA.