iPhone.... I am dead serious Verizon has one of THE best networks in the entire country.... sure it's not ideal for travel, but really the reliability of Verizon is amazing and their speeds are among the best I have ever seen on a CDMA carrier. Besides the random LTE shutdowns, it is a really reliable service, and for sure you can expect AT&T to have some issues with their LTE network when it starts up. In the end though you have to look at what you need on the phone and where and how you will be using it.
iPhone 4: basic smartphone needs, iOS really dampens the word "smartphone" however the network is reliable and just overall going to last longer( keep in mind AT&T and T-Mobile might merge) however the phone is really out dated and here at the Android Central forums, iPhones are for sinners xD
Nexus S: Android offers a whole complex operating system which in the end meets power users AND basic users needs quite well. This is what I believe is the DEFINITION of "smartphone" it is a great phone and the operating system is great for a power user such as myself the T-Mobile network is probably the fastest 3g networks out of the big 4, but they were one of the last networks to start effectively pushing 3g, so their coverage is really just flat out terrible compared to Verizon. Plus there is the possibility that T-Mobile will be merged with AT&T ( I don't think so because the majority of the cons definitely outweigh the pros)
What should you pick? I would say it is your decision buy I have not a clue why you pinned just these two devices, Verizon was one of tthe first carriers to start pushing Android and it has been a success, they have the most Android devices out of any other carrier.
My choice for you? Droid X2, big screen, nice cameras, Android 2.2( it's something), Motoblur( this is great if you are a social networking addict, it may look not as nice as other UIs) it offers everything Android and Verizon, sure you don't have 4g, but as of now the network just isn't stable enough to actually want it.
Good luck in choosing your next device. Hope I helped.
Sent from my Nexus S