There will never be a way to compare. Verizon is CDMA and AT&T is GSM, two completely different cell systems. Not only that, the iPhone for AT&T was never optimized, but rather was equipped with a semi incompatible chip. Therefore the iPhone dropped calls and generally never operated very well as a phone.
I carried both an iPhone (every model from day one) and a BlackBerry (for work) every day since the iPhones were introduced. The BlackBerry _always_ performed very well on AT&T as a phone. Clear, great voice quality, and volume, never a problem. And I'm _NOT_ bashing the iPhone. It's simply a fact. The reason so many iPhone users bash AT&T is they simply don't know any better. Then combine that with the victim culture of Apple where pointing fingers is widely accepted as a way to place blame on anyone but Apple and you have what you have. AT&T hating masses.
Apple is not about to make the same mistake again, they will build a better phone for Verizon.
Oh... and by the way I'm a customer of both AT&T and Verizon, so I'm not sticking up for AT&T. They are both good networks, I'm a huge smartphone enthusiast, and having Verizon allows me to enjoy the Droid X and other phones they've offered over the decade I've been with these two companies. But this in no way lessens the bad coverage in Des Moines, IA where I live. I never had these problems before I went to "smart phones" Hell, my Motorola MTAC worked better than my 3G, 3Gs, iPhone 4 regarding dropped calls. My old Nokia's, and ancient OKI worked better than any iPhone regarding dropped calls. I have no idea why. Of course they didn't do data. But I have to ask myself why my old cell phones dropped calls with less frequency than my various AT&T iPhones?
Finally, I really like my iPhone...and I don't care if it's not a good phone, because everything else it does... it does very well. At the end of the day my iPhone, MacBook Pro, Mac Pro and other Apple gear are all terrific....