I don't think I'll have problems handeling the 5.5", but since my current iPhone 4S has a great compact size, I might just go with the smaller 4.7" version. Guess I'll have to see one in real-life to decide.
Maybe the 5.5" has greater battery and processing power (or a sapphire display). Maybe even software enhancements. That would make it very difficult to choose.. Unless the price-difference is very big.
I will take the 4.7 as my daily driver. Been waiting for years for a lager iPhone but not ready for a "honker" 5.5"! I might buy both but can not imagine putting up with that large of a phone on a daily basis.
What is great is Apple is giving us choice which is always good . . . . well not always as some people are not good a choices.