depends what you want.

github is quite ok for public projects and you can have a lot of collaborators, but you have to pay a licence when you want to hide the project from the public.

with bitbucket you can have private repositories, but only 5 collaborators without paying a licence.

If you want to spend some money, you can also go to Jira directly (Jira owns also bitbucket).

Best 

What about: https://r-forge.r-project.org

Seems to be dedicated to using and developing R packages.

depends what you want.

github is quite ok for public projects and you can have a lot of collaborators, but you have to pay a licence when you want to hide the project from the public.

with bitbucket you can have private repositories, but only 5 collaborators without paying a licence.

If you want to spend some money, you can also go to Jira directly (Jira owns also bitbucket).

Best 

R-Forge has the advantage that it checks and builds your package in different platforms and you can submit it directly to CRAN. It is totally oriented to build R packages and it uses SVN instead of git. However, in my opinion, it is not very confortable to work with, at least compared with github or bitbucket (with the differences appointed by Matthias Templ. So, if your goal is not to build a package and just maintain a couple of scripts, maybe git would be better.

I agree with Andrés. If your goal is to create a new package, you should use R-Forge.

But if your goal it's to develop some scripts with some people, or alone but using versioning, I would recommend using GIT, specially Github.org

You should check gitref.org for a quick intro to versioning.