@clacke R5RS doesn't have library/modules, so it's unlikely to be long-lived. R7RS-small has bad decisions like abandoning Unicode, and R7RS-large is a bloated piggy, it'll never be done or implementable.
The two most solid impls of Scheme (Chez & Racket) are R6RS (Racket also has its stuff on top), and probably will be for forseeable future/maybe forever.
Maybe R8RS will sort that out next decade. Until then I just simplify my R6RS code if I need it to be more portable.
#scheme