iPhone 3G - Same specs as 2G except 3G radio. Only got updated up to iOS4 and was awful.
vs
iPhone 3Gs - First major spec boost faster SoC, double RAM video recorder by default. Got up to iOS6.
iPhone 4 - First major redesign, spec updates, but SoC was still single core and aged badly by iOS7.
vs
iPhone 4s - First to get dual core and aged well up to iOS7 and iOS8.
iPhone 5 - Another redesign with no major drawbacks like the 4 except scruffgate while being the first to get 1 GB RAM but....
vs
iPhone 5s - First to get Touch ID (home button looks better), 64-bit processor, M7, slow motion, much better camera, etc. Space gray > slate for less scruffs.
iPhone 6/6 Plus - Best iPhones ever made (6 Plus) but only 1 GB RAM... iOS with more features will start to become more like Android (useful) but also bloated.
Looking back, alot of people who bought an iPhone 4 wanted enough reasons to justify their purchase and find enough reasons NOT to buy the 4s. I remember so many people bashing the iPhone 4s for having the same design and GSM Arena making fun of the name (forest or for a**). Yet, the 4s while looking the same aged better than the 4. The iPhone 6s/6s Plus will get the same maligned comments although this time more expected as most people thought the 4s would be called iPhone 5 with a different design back in 2011.
If you have the same looking models, always go and keep the faster model INTERNALLY for the longer term. Look at the track record above. Yeah, the design is no longer fresh. Apple doesn't want to be forward thinking with the tick years. They always want to hold back something for next year. The s models is for people patient enough with some enough foresight in keeping the models longer.