In terms of Streaming Twitch is a bigger platform than YouTube but there are many other factors that you need to keep in mind. Location is one the most important term to decide, Twitch has more viewers in countries like US and UK. But on another hand, in Asian countries like India twitch is not so popular. YouTube content is too broad and so the viewers but most people visit Twitch for enjoying streaming, You must stream on Twitch and share your past broadcast on YouTube by enabling VOD on twitch.