If you've not used a five for any length of time, and the deals are a lot better on the four s, then go for that. If you've lived with a 5 for a while the 4 seems too small afterwards.
Battery life on any modern smartphone is about a day.
Any modern smartphone will have an acceptable camera, most apps you'd ever want, and a nice screen.
Android phones allow more customisation
iPhones are better with email (exchange email anyway)
If you just want something you can pick up and use, and does what it says on the tin, then IMO, go for a 4 or 4s on as short a contract as possible, as in 6 months or so there'll prob be another iPhone out.
It's rumoured there may be a cheaper version out soon too.
Basically an iPhone 5 internals, in an iPod touch body........will cost less, so may be worth holding off for a while.
After the initial month of new fangledness and whatnot has worn off you'll find you use it for calls, texts, email, and reading the oof on the toilet, so go for the best deal, and not the shiniest phone.