It depends on where you are. I've been using verizon because they are the only ones with decent signal in most places in my building, and the whole point of a cell phone is that I have it available as much as possible. So first thing is check with your AT&T friends and your verizon friends how is their coverage where you are spending your time. If it's good then you move to compare the secondary options.

Smart phones are fun, think of them as a small computer that you have available everywhere. You have internet, maps, gps, pretty much anything you want to access available right when you think of it. So, if you can afford the data plan go for it, you probably won't regret it. I also think within the first 30 days of your contract you can cancel without penalty, afterwards it becomes rather expensive to terminate, as they give you the equipment at a discount and make money on the usage (gillette blades model).
if you don't want to

iOS vs. Android is largely a matter of style preference. iOS is much more closed and restrictive and there is a single device, but overal is sleeker.