I think the 4S is going to be the best bang for your buck It seems like it's the same as the 4, but it has a better processor and better camera. The camera doesn't seem like it would be a big deal, but it has completely replaced my point-and-shoot. If you use a point-and-shoot camera yourself, you may want to consider the camera in the iPhone you get because you'll probably end up ditching your regular camera like myself and most other people have.
Also, if you're the type who isn't going to upgrade iPhones with every new model or even every two years, a newer model is the way to go since it will be supported longer by operating system and app updates. In that respect, while you would get more mileage out of the newest model, I think the 4S is the real sweet spot for value.
2GB should be plenty if you're not streaming a bunch of music or movies. Plus, you can use Wi-Fi like you mentioned. Between home and work, my data use comes on Wi-Fi about 90% of the time. If your phone is not going to be connected to a Wi-Fi signal most of the time though, it may be something that you want to toggle on or off in the settings on your phone. If you're on Wi-Fi most of the time like I am, it can be a bit of a battery saver, but if you're off Wi-Fi a lot it can be a drain if you don't turn it off because it will keep searching for a connection. My wife doesn't have Wi-Fi at work, so she leaves it turned off even at home to save her battery. She doesn't stream music or video and doesn't come close to using 2GB, even though all of her data is coming from our carrier.