TAing at Berkeley is supposed to take 15-20 hrs/wk and Stanford expects you to teach a 1hr disc and a 2hr lab section, although I'm not sure how much prep that would entail.
Stanford TA duties might take slightly less time but not a lot because once you teach one discussion/lab section, a second is hardly more effort. Figure in grading and preparation.
I don't expect the workload to be significantly different between the two as this is somewhat standard across the board. As for cost of living, the straight up numbers have Stanford being way more expensive to live in. It's a very yuppie, wealthy area whereas Berkeley still has that college town feel. There lots of good, cheap food near Berkeley.
Stanford is more isolated and spread out. You'll miss not having a car a lot more than at Berkeley, where most of what you need is within easy walking distance. Stanford is about 45 minutes away from San Francisco by car, Berkeley is about 20.