I would ask friends in your area of use about their experiences with AT&T. For me, switching to AT&T was the best thing I ever did. Many places where my Verizon phone never worked now work fine. I also have yet to find a place where it doesn't work for me while on the road. I had one dropped call in 6 months but I also had them with Verizon occasionally.
The quality of service varies all over the country for every carrier. You need to research what will work best for you in your primary calling area.
I still think the iPhone is the best smartphone around. All the new ones from the competition are improvements but still look like imitations to me, and are missing some important features.
The ease of finding and installing third-party software is what made the iPod the most successful mp3 player, even if is was never the best player. The same goes for the iPhone in comparison to other smartphones.