I never really experienced the battery problem because I use WiFi at home and when I'm out, I tend to turn off 3G unless I'm actively using the internet on my phone. This makes the battery last around 7-12 hours for me, which is very good indeed. I recommend trying this out and seeing if there's any difference.
iOS7 is absolutely beautiful in my personal opinion and I feel opposite in terms of hardware capabilities. Apple have a great way of pushing updates to their mobile platforms with minimal hardware upgrades. While iOS7 may need more processing power, it also comes with new technology that tries to use as little processing power as possible and still improve battery life. The A5 chip in the 4S should be more than capable of handling iOS7 with ease. It may be a different story with the iPhone 4, however, which I believe will start to feel the strain from the upgrade.
Is the new iPhone going to be different? I'm not too sure. Apple are unpredictable as hell - one of the reasons I'm particularly fond of them from a design and decision making point of view. On one hand, it would be safe to make things look similar but have the next iPhone have improved functionality (as is the pattern with 4 and 4S). The design overhaul would have been with the 5 and the improved insides would be the next one or "5S" as people are calling it. To me, the 5 is beautiful anyway so if they don't change the design this year it's not going to stop me from getting the next iPhone. A radical change of design is only a plus, really.
That being said, Apple is still a business and from a business point of view the smartphone market is in such a competitive state right now that maybe they can't afford to just overhaul the insides in order to attract more customers - hence the (justified to an extent) rumours of a cheaper iPhone "5S" being released alongside the new one. Same insides, OS, etc. it's just made out of plastic instead of glass and metal. For me, the whole point of having an iPhone is the premium materials that it's made out of - therefore I'll still rather get the other, more expensive, "5S". The cheaper model could prove to be a game breaker in the smartphone market, however, as it means anyone has access to an iPhone now.
Try turning on 3G only when you use the internet on your phone and closing the apps you don't use manually (double click the home button and hold down the apps until they start to wiggle) and it does make a difference - especially not having 3G on every time.