I know that once in a great while you might use data and voice at the same time, but how often does that really come up when you are not near a PC or WiFi connection?
I'm in Northern California, I currently have ATT w/ unlimited data and I'm planning to go to Verizon because I'm just tired of the poor coverage or performance with ATT after having previously been a Verizon user for 11 prior to making the iPhone jump to ATT.
I'm blown away that people say they can get above (even well above) 1Mbps download on their ATT iphone. The highest I've ever gotten in the 2 years I've been with ATT is 0.8 Mbps, and the average is about 0.35 Mbps. I noticed latency is very high with ATT (often around 200-300ms). I was also tired of so many data dead spots in the downtown city where I work. I'd have 3 bars, but data would be a crawl or non-existent. Couldn't even send an email if I wanted. Because of my frustration with the ATT network, when I bought an iPad 2, I went with Verizon.
With my iPad 2, I have better signal strength, and I notice the latency is always below 100ms on it with Verizon. Doing a side-by-side Speed Test with the iphone (ATT) and ipad2 (Verizon), the ipad is either near or better in terms of download speeds. When loading webpages, ATT always hangs for a long time before ever getting the page to load up. Verizon is very quick at that.
So now I have a Verizon iPhone 4 coming in the mail with the unlimited plan, and I'm about to retire my ATT iPhone 3GS w/ unlimited. The ATT technology is better, but I'm not seeing the performance.