When you’re deciding which iPhone to buy from the new line-up, your decision will be driven primarily by what size screen you want. The iPhone 5c (sadly relegated to one capacity, 8GB) and 5s have a 4-inch screens, while the 6 and 6 Plus have 4.7 and 5.5-inch respectively; think about how they fit into your life – literally. There are other differences; the 6 Plus’s camera has an image-stabilising lens, and both 6 models are faster.
The iPhone 4s has sadly been retired by Apple but there are still some available from carriers (and second-hand, of course), so if you want that 3.5-inch, thumb-friendly design, you can still get it. The iPhone 4s will run iOS 8, but don’t be surprised if it won’t be upgradable to iOS 9.