This shouldn't even be a question. I live in the DFW areas and visit Houston frequently and Houston is an absolute sh*t hole! It's completely flat, swampy, awful weather, mosquito infested, drab city. There's just nothing good there. The depth of runners is poor. The Woodlands has some decent runners but other than that it's being controlled by hobby joggers.
Dallas on the other hand still sucks, but it just sucks less. It will be less humid in the summer months and will actually get some snowfall in the winter. If you can I would advise living more towards Fort Worth. The running community is better there, the trinity trail is a fine place to do a long run, plus many other good trails. Lukes locker and the 'Texas running center" are also based from there. Both cities are pretty shallow for running considering how large they are, but the DFW area sucks less in all areas.