Short answer: You probably need neither of the two. If all you want to do is develop software for the iPhone/iPod Touch, you only need a Mac + XCode + the iPhone SDK.

Long answer: The iPhone SDK provides everything you need to test your application, but there are things to consider: the iPod Touch is slightly more powerful while the iPhone has features like GPS that have only limited support on the iPod Touch. But it really doesn't matter which one you use for development. If you already have a 3G phone, go for the iPod Touch or only the simulator on the Mac. Remember though that you cannot develop apps without a Mac. And in order to get your apps onto the device you need to pay as well.