I currently own a 4.8m Ribcraft with a 60Hp EFI fourstoke outboard, and can say that for its size it is a good Rib.
Considering the price difference between a 60HP and 50HP I would go for the 60HP. Take a look at the Mariner / Mercury technical forum where they get many enquiries regarding what they need to do to make there 50HP a 60HP!!
The extra power can be usefull when the Rib is heavily loaded, in rough conditions or when pulling a water skier etc.
However, with 50HP on a Ribcraft 4.8m you can still have loads of fun.