iPhones.. Expiry dates?

Mhmm. I currently own an iPhone 3GS and an iPad 2. I don't really 'want' a new iPhone, but I almost 'need' one.

My 3GS is awfully slow. It's volume switch isn't functioning. In short, I can no longer stand it.

Question is, can I expect to get 2 years of life from a 4S without it being like my beloved 3GS?

BTW: We don't do contracts in New Zealand, it's all prepaid mostly, hence the question.