The age demographics are similar for both cities, with about 40 percent of the population between 18-44, but my feeling is that among that group, San Antonio has a much larger percentage of people who grew up in the city. Many Austinites in that age group grew up in other cities around the state and first came to Austin for college. Austin has a slightly higher median income and quite a bit higher average level of education (because of size of UT and the many students who stay, Austin is toward the top of the list nationally in that category).
As many people have mentioned, San Antonio is HEAVILY Mexican. About 60 percent of the population according to the census, and that's likely vastly underreported, as many residents, both legal and illegal, won't fill out census returns. (I realize I'm lumping all Hispanic backgrounds under the title "Mexican" here, but that's far and away the dominant ethnicity.) Turn on the radio and there's about a 50-50 chance the language you hear will be Spanish.
The main entertainment area in San Antonio is the Riverwalk, but it's pretty different from the areas I mentioned in Austin. Whereas Sixth Street, the Warehouse District and Red River are mostly bars and clubs with a smattering of restaurants, the Riverwalk is more along the lines of a restaurant-heavy tourist destination.
If you like Mexican food, you'll find some of the best in the world in San Antonio. My wife's sister lives there and there are a couple of places I make sure to eat at whenver I have to go down there. But I'm always glad to leave. The whole city feels to me like one of the areas of Los Angeles that aren't TOO bad, but feel like they could get that way in a hurry.
