Techware Labs has posted up their side by side comparison of the iPhone and iPhone 3G.

Quote:

"So your probably sitting back in your chair wondering “Why would you buy the new iPhone if you already have the old one?” Don’t worry, I’ve asked myself the same question. The lower price of $199 makes the iPhone extremely appealing to impulse buyers, however the additional $10 a month for the 3G iPhone plan cancels out the reduced price. Additionally the price of $199 is only available to those that are eligible for an upgrade, already own the original iPhone, or to those that want to sign up for a new contract. If you are stuck between a rock and a hard place, much like I was, on whether or not to upgrade from the older first generation iPhone to the new one, here’s my advice - don’t do it. The only new features are GPS (not even a real turn-by-turn GPS), and the 3G speeds, which eats up your battery. If I hadn’t signed the 2-year contract already with AT&T, I would go back to the old iPhone and save the $10 a month.”

They seem to do a nice job of making the rundown of the two units, give it a read: iPhone vs iPhone 3G @ Techware Labs