I'm currently using Python to teach someone else programming, for the lack of a better language. The first thing I did was teach the person what lexical scoping is, and show how broken it is in Python. Things seem to be going pretty well. Currently we're doing recursion (with pretty advanced problems) and I still haven't explained what for does.
The current Python runtime is probably "objectively" faster than the Ruby one, but realistically, you won't use either if you care much about speed.
The choice is more likely to come down to wanting to use frameworks or libraries from one of the other, which again is based a lot on personal preference.
I'd say Python is better suited as an introductory language, because it is a fairly small, simple and regular language, but that's my personal preference shining through. Ruby on the other hand inherits some sugar and spice from Perl, which might appeal to a Perl programmer who for some reason didn't want/get to write in Perl proper.