The main difference is that Screentop runs in a web browser. To start a group play session, you can just send someone a URL and there's no need for everyone to purchase / install / setup TTS.
This also makes it multiplatform. While (paid) TTS supports Windows, Linux and Mac, running on a web browser lets you play this (presumably) on an Android tablet, Chromebook, or iPad.
Loads fine on my iPad and can’t play due to lack of touch, but came here to say my suggestion is allow users to tap the legend items (“Press space to flip”, for example)