I don't use Twitch
I don't know the other one
With streaming, however, definitely Twitch - YT's streaming is rather poor, I think.
Twitch to stream. YT to watch videos.
But you have the advantage that YouTube needs a little less internet power than Twitch. And since I have a maximum of 5mbit myself, I'm glad that I can watch stream in 480p on Twitch at all. At 720p or 1080p, I can forget because that stutters, stutters, and stutters. It looks different on YouTube, where I can even watch a 5mbit live stream with 720p without any problems. So unfortunately Twitch is a little less optimized for Internet performance than YouTube.
Because of my bad internet I only use YouTube to watch videos instead of Twitch.