but takes a rather long route with many stops, so going to downtown San Francisco from SFO is not actually much faster than the AirBART bus + BART from OAK.
I disagree with this. THe AriBART bus runs every 15-20 minutes, so you have to wait for that, ride the bus, then wait for a train (which is not coordinated with the bus, mind you). So you could potentially have 35 minutes of waiting time in addition to the bus and BART rides.
Compare this to SFO where you could wait up to 15 min. for a train, and I would definitely say advantage goes to SFO.
With that said, OAK is perfectly doable with transit. It's just a bit more of a pain.