Hola Edoardo!

Well, I haven't TTS-1, but as far as I know it's very similar to the Roland family of Sound Canvas and Hyper Canvas: the look of TTs-1 is exactly the same of Hyper Canvas, so it's likely that the sounds too are very "near".

Vi One, in my opinion, gives you something more as sound quality. It's also full of hundreds of different instruments, but inside BIAB the only logical way to use it is by the GM module. This means that all the other sounds of VI One, many of them really great, aren't available (as far as I know...) in BIAB. Of course you will have them, if you want, for further editing with different DAWs.

Ciao!