Virtualenv is a nice tool for managing library sets.
RVM allows you to do that plus it allows you to install and manage multiple versions of Ruby itself. Want to have side-by-side Ruby environments of 1.8.6 and 1.9.3? No problem.
Interesting, I've had an almost opposite experience, but I don't know the ruby landscape all that well (bundler/gemsets/rbenv/rvm etc) and I suppose it just depends on what background you're coming from.
FWIW, when creating a virtualenv the -p flag lets you choose a python binary (and therefore version).
rvm will download the requested version's source, apply any patches or flags you specify and then compile it. You don't have to do any work other than installing rvm. The rubies are also stored in ~/.rvm so your system doesn't get cluttered up.
Rvm also integrates into your shell so that when you cd into a project directory with a .rvmrc file, it will source the file and load the proper ruby and gemset for that project.
Virtualenv is much closer to ruby's bundler gem than rvm.
