Quite simple really, we currently have the iPhone 3G that we purchased when released and have served us well for the most part. The 3GS is a little old and most of the current software is written for the 4/4S. The iPhone 4 had some issues that were resolved with the 4S, and the camera is better with the 4S than the 4, so after all that the iPhone 4S will be the best choice for now and the next few years. Had there been no major changes between the 4 and 4S such as between the 3G and 3GS, I probably would have gone for the iPhone 4. The price of the phone has no effect on my decision.
*Edit - I should correct myself by saying the only real tech spec that I care about is the camera. The rest of the stuff is great I'm sure and people use their phone in different ways and the processor, screen size, on board memory, etc. may suit them for their purpose. I'm not really a power user when it comes to phones.