If you have a smart phone, you will be able to do a lot more, but expect a higher cost per month. Smart phones are really good at being able to install additional apps that can save money, and save effort. You will also have a wider range of options to make your phone do more for you.
In my instance, I have a smart phone which I use to get bus times for the local public transit system. I also have Facebook, and Twitter as well as the less popular Myspace. I have a security software to have a better chance of recovering my phone if it got stolen. I have Pandora which is like a custom radio station, and use Google Voice for voice mail, and SMS which comes to me as data. All of this will require an internet connection. There is of course a full HTML browser (on a 3.2" screen), and email client as well. But those are some examples.
If you just want to make and receive calls, you probably don't want a data plan, nor maybe even a smart phone. However, if you want your phone to be more useful, a data plan and a smart phone is pretty much a requirement.
Examples of a smart phone are
All Android based cell phones
All RIM (Blackberry) devices.
All Windows Mobile phones (not PDAs)
All Windows Phone 7 devices
All Web OS devices