There are no practical differences between them in regards to Robotic development. Using VS2015 with the Plug-In allows you access to all of the features of Visual Studio whereas the Studio version is just a free shell version of the Visual Studio Shell and does not have things like code suggestions and what not. If you are a developer and have a license for VS 2015, you should install VS2015 and use the Plug-In. In terms of making things different from strictly Robotics development, there won't be anything significant.