I applied and was accepted to GaTech and UT Austin as well (but did not apply to Caltech), so here are my thoughts.
Caltech - top-notch faculty, but the department is extremely small (both the theory group and the overall CS department -- on the theory group website, apparently there are only 4 grad students!) so I would not even consider it UNLESS you are interested in quantum computing (since Caltech has Kitaev, Preskill, Schulman, and perhaps others, while I don't believe anyone at GaTech or Austin does anything quantum-related).
GaTech, UT Austin - similar in many ways: good weather in mid-size towns, overall large schools, similar prestige levels, great theory faculty (though GATech has a larger theory dept, so maybe is a little better in this respect), GATech has a new CS building and UT Austin is building one soon. Personally, I would choose UT Austin (since I liked Austin the city a little better, I thought the other prospective students were a little stronger, and UT Austin gave me a higher stipend), but it seems really to be a coin toss.