It's helpful to think of Startup School as a conference -- you don't really go for the talks, as much as the people you'll meet between sessions. If you come with that purpose in mind, the experience can be immensely valuable (My first SS was the reason I moved to San Francisco!)

That said, I've never been to SXSW, but my second-hand impression of the event is that it's less focused on startups and technology, and more generally about promotion, business development, schmoozing and...well, music. It's primarily a music/arts festival, with an "interactive" component tacked on. But it's also much longer, and will fill up more of a week's trip.

My advice is to remember that these are social events, and to choose appropriately based upon your interests. Like any other convention, the talks are just a reason to get people together.