If you (and the co-maintainer) already have a source repo that meets your needs (especially if it is publicly available), I would just stick with that. Just gathering bug reports, IMO, is not reason enough to push an unmaintained copy (assumption) of your source to github. IMO, it is better to have one canonical source, and use the rest as automatically updating child repositories. Changes can flow up from the child to the parent, but without the automatically updating (or just a lack of communicating that XYZ is the master repo) part, there will be a question as to which is the authoritative source.

However, if you are planning on setting up synchronization between the two repos, or if RT does not meet your needs, or if GitHub provides some whiz-bang feature that you just need to have, then it may make sense to look at GitHub for hosting your source.

GitHub also supports a subset of svn operations.

There is more value to GitHub than just that it uses git. There are pulls, issue tracking and management, project wiki, and a few other tools that I am not familiar with. It comes down (at least to me) to if the tool does the job you need. It seems to me that if the provider of your source repo is doing everything you need, change for the sake of change is not a good thing.