* Multi-lingual: projects w C#, Python, SQL, etc.
* Multi-line repl: recall/edit/submit chunks of code
* Lots of add-ins: like 6,000 (hopefully a few are useful for R!)
* Upcoming integration w Excel, Azure ML, SQL, etc.
* etc.
If you are already a VS user, checking out RTVS is a no brainer.
But in summary, RStudio is better :). Hopefully we'll have some key differentiation features in upcoming releases.