I'd say just go with whichever language you find most comfortable. Both appscripts provide the same level of functionality, and the APIs are pretty much identical modulo the variations needed to accomodate the different languages' strengths and weaknesses (e.g. Python has better keyword argument support, Ruby has a proper Symbol class).
Python appscript has a few nice add-ons, in particular its built-in help() method, which I've still to implement in rb-appscript. However, you can already get most of the same functionality via the osadict tool which is bundled with py-appscript and provides support for Python, Ruby and AppleScript, with more languages planned.