The plus version has 6 new multiplayer levels, and also includes the multiplayer levels from G-Switch 1 and G-Switch 2. It also has more multiplayer options, such as choosing which levels to play and in what order. The single-player content remains the same.
Also, compared to the web version, this version has higher-resolution graphics, full-screen mode, and runs more smoothly.
Let me know if you have more questions, happy to help. Cheers!