Just keep in mind that the next iPhone is going to be a LTE iPhone so 3G speeds and coverage should be a much smaller factor in your decision on a carrier. At the moment, Verizon's LTE coverage is much better than ATT's coverage. Not to mention, according to 3rd party analysis, Verizon's is faster by a couple mbps.
With the new facetime ability over cellular (LTE) and the main browsing experience will be over LTE, that should be your major concern when you're deciding on your carrier in your area. Given, if you have a solid signal for voice over 3G.
That said, have you looked at how ATT's true LTE fairs in your city? Is it even deployed in your city? HSPA+ was proven to be quite a bit faster than Verizon's 3G but even hspa+ can't compete with true 4G LTE. With the iPhone 5 going to be a LTE phone, as long as you get a solid 3G signal for voice, data is going to be sent exclusively over 4G, with the exception of deadspots.
Check this review out:
http://gigaom.com/2012/04/14/solving-the-lte-puzzle-comparing-lte-performance/
It's an average of Verizon vs ATT in 15 markets.
Coverage
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Full Signal Speeds
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Real World Averages
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TL;DR: The iPhone 5 is a LTE device and you should see if ATT or Verizon has better LTE coverage (true LTE not HPSA+) in your area. My bet is probably on Verizon for coverage but ATT for speed, though only time will tell.