A big one, yep. Synthesia is a software product while Pianu is a website.
Pianu requires no download or installation, and works on any computer connected to the internet.
Synthesia, on the other hand, must be purchased and installed on your computer's hard drive.
Pianu also has a growing library of ready-to-play songs, while Synthesia requires you to provide the MIDI files of each song you want to learn.