The iPhone is as durable as the user allows it to be, meaning that if you're careless with it, things happen to it but if you take some care (don't keep it in the same pocket as your keys, take it OUT of your pocket before you throw your pants in the washing machine and don't drop it out of a 4 story window simply to see how high it bounces) then it should be fine, but in general I would advise you to get the iPhone 4.
First of all the 4 is rumored to be a lot more durable (the glass on the back and front is said to be the same glass helicopters have to stop bird who crash into the chopper from entering). It'll last for a good 3 years easily that I can promise you. My iPod touch has been with me for a few years and never got a single scratch on it without the case until I started being careless with it. I take the iPod with me EVERYWHERE and it has never failed me yet. Apple products in general are awesome
My cousin has a 3G (he's 18 for the record) and it is the ONLY phone he has not broken to date. It has taken a fair beating from him dropping it, kicking it (don't ask) and basically doing things with it when he wasn't sober and still works perfectly. He even brought this case for it that makes it a bit bulky but more secure than Guantanamo bay. Apple stuff lasts, and I'm living proof of it. I still have an iPod mini, the first generation and the first iPod I ever bought that works like a charm.
But back to the point, get the iPhone 4. The 3GS will last you physically, but it'll be outdated in a year easy. the reason is that it lacks a heavy hardware upgrade that the iPhone 4 has had and all the next gen iPhones will have as well. The iPhone 4 uses a faster chip, a gyroscope and a few other goodies that the iPhone 3gs doesn't pack. Soon the OS won't be compatible and you won't be able to use half your apps with the 3gs.
The iPhone 4 is also a lot more durable. With a protective layer behind it as well as a stainless steel border, it can take a few hits (will be getting one tomorrow and doing an unboxing on youtube if anyone is interested, my youtube username is the same as the one on this site) though I haven't tested it, Jobs does stand by that. Also, the iPhone isn't exactly a wimpy device.
DON'T TRY THIS AT HOME: One of my good (but slightly slow) friends' iPhone 3G fell out of his pocket right behind the wheel of my other friend's nissan rogue. Before we realized it, my friend backed out of his parking space and ran over the phone. After we got it, it had a crack down the middle of the screen and a lot od edge damage, but still worked (with a few dead pixels) and could receive and send calls and texts (though the camera was shot). These things will last as long as you aren't careless with them, and the iPhone 4 is supposedly the tougher phone so think about that for a second.
The one thing that did actually get one of my iDevices however was water damage. UNDER NO CIRCUMSTANCES SHOULD YOU TAKE THE IPHONE IN THE SHOWER. Fair warning. I will neither deny or admit that I felt like showering with my iPod one day and to this day the click wheel doesnt work because of it. again I will neither admit or deny it.
Anyway bottom line, GET THE IPHONE 4. :D