Austin has a better nightlife, but there's lots to do in Dallas.
If you decide on Dallas, here's some apartments I'd recommend:
http://www.reserveatwhiterock.com/si...TOKEN=79721253
It's right next to White Rock Lake (the best thing in Dallas, IMHO) with a 12 mile bike path.
Here's a map of the neighborhood:
http://maps.google.com/maps?q=9215+G...i=map&ct=title
There's a gym right across the street (24 hour fitness, I think), and a really good no-nonsense hard-core gym about 2 miles away. It's a short drive to downtown Dallas, and not far from upper and lower Greenvile Ave & entertainment (that's where all the straight folk go, anyway).
As far as renting a $600 apartment in Dallas -- no, I wouldn't recommend it. You could probably find a really good deal somewhere for that much in Dallas, but I wouldn't count on it.
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I've lived in several parts of the Dallas area. It's pretty big, lots of different virtues in different areas. Do you want to live close to work, or close to play, or close to a good gym, or close to an airport, in the 'burbs, or just somewhere cheap?