As a developer, I had the chance to work with iOS 4.0 for some time before the general public, so here are some thoughts:
First, just because something is newer doesn’t mean it’s better, as anyone who watches American Pickers knows.
If you have an iPhone 3G, you cannot use many of the new features in iOS 4, such as multitasking and even wallpapers. Obviously you cannot use FaceTime and iMovie as the iPhone 3G doesn’t have a video camera or front mounted phone, so you get very few benefits by upgrading your operating system.
What’s more, many users have complained of slow overall performance for an iPhone 3G running iOS 4 compared to iPhone OS 3.1.3. You might be better off reselling or gifting your iPhone 3G and upgrading to the iPhone 4. There's an interesting video comparison here: http://gizmodo.com/5569969/iphone-3g-owners-you-might-want-to-hold-off-on-ios4
iPhone 3GS owners can leverage some of the benefits of iOS 4.0, such as multitasking, but it will require updated apps that support multitasking.
Of course, all iOS 4.0 users can watch ads using Apple's iAds, so you've got that going for you.