I always upgrade every 2 years and not when 4s, 5s is released (which I believe is the best to do)! But if the phone is two big I might consider other solutions...
The s is always the best version to go for as its the Upgraded model, I think if there is a 5'5 model and 4'7 I would consider the 4'7 after seeing it in person instead of ordering it the second its available online.