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According to Google's Adwords Keyword Tool, "austin plumbers" gets more search volume (go for Match Type 'Exact' in the left-hand column). Neither offer much volume though, so the results might not be that accurate. In your title tag (and everywhere), I think you should use whatever makes most sense for the user, with SEO secondary. Your text should look and read naturally otherwise it'll come across as spammy. It's good to use variations of your keywords, and not always right next to each other, e.g. "plumbers from Austin". Take a look at this for example: http://www.seomoz.org/blog/perfecting-keyword-targeting-on-page-optimization - it's an old-ish post, you might not want to include as many repetitions of the keywords - natural text is the key.