Do I need AT&T Navigator service if I get an IPhone with a GPS application?

I was pricing out the IPhones from AT&T, and looking at all the services they provide. The AT&T Navigation service is .00 per month, but in the ITunes applications they have a (so called GPS) program that you can download straight to you’re IPhone. What’s the difference? Do any of you have any experience with any ITunes navigation applications? It would be used for basic traveling on freeways and such. Thanks in advanced.