I think what is important for the iphone is the system. To have the latest version is important especially for apps. About the device, it depends on where you are. If you are in a country like US, it is better to have the 3Gs because you can enjoy...
Yes, I have used an iPhone for an extended period of time and while it's a great consumer device, it's just not strong enough when it comes to business. It's missing true enterprise push so messages take too long to receive unless set to constant ...