I actually went through all of this myself. I live in Central, Louisiana and when Verizon got the iPhone I decided to give them a try. I didn't port my number over though because I was honestly unsure of their coverage and heard good-bad reports. Around my office and home the coverage was actually pretty good with Verizon. I did notice their 3G wasn't as fast as AT&T's 3G. This became pretty evident right off the bat. Then I started traveling in areas where I've always had service with AT&T. Low and behold for 32 miles going west of my city there was absolutely NO signal. I was astonished. Then another day I traveled south and for about 28 miles I would get 1 bar, searching, no service, 1X, expanded. But I had no internet and actually couldn't make a call. YES, I tried!
Needless to say in those same areas my AT&T iPhone 4 always had a signal of 2 bars or more. I did hit like one area traveling south where it lost the signal momentarily, but came back.
The next day I traveled North and I noticed in an area about 12 miles that AT&T had no service and Verizon had 2 or 3 bars. I was well okay... this is tit for tat.
Now to make a long story short after a good solid 10 days of testing, I ended up returning the Verizon iPhone 4 and discontinuing the service. Unfortunately I travel a great deal west and south of the city. The good news is that AT&T recently bought out a local regional carrier and in the process the FCC made them diverse of a few markets. Most of those markets are south of the city and AT&T sold them to Verizon. So that should improve Verizon at some point going south. The real deal is though they have no real plan at this point except for a long term one to add towers to provide service west of our city.
Right now AT&T provides a really good level of service in the city and in the rural areas I travel. Ultimately that is the ONLY thing one should consider. I am sure Verizon is better than AT&T in other markets, but not this one. I can live with a 12 mile stretch going north of the city, since I rarely travel that way.
On the main highways and interstates though I'd say AT&T and Verizon are comparable. Though there were areas where AT&T only had Edge and Verizon may have had 3G. I think it is true that Verizon has the larges 3G network, big deal if you don't have coverage and signal where you live and work the most.