I think it all comes down to whether you want to appease to your audience or grow your audience.
Youtube streaming is not well promoted and it will not be easy at all for people to find your stream unless they are subscribed to you. So you will not grow by live streaming, just make some of your subs happier.
Twitch streaming is a completely new audience. Every time you stream there is a good chance you will gain new viewers. Even being at the bottom of the pile will get you some new viewers. This does not make it easy though, you have to work and if you ever want to get big on there you need to schedule. Twitch do not let you partner with them without 500 views at least a stream with you streaming at least 3 times a week. There is exceptions to this rule but for the most part just see it as this.
The fact that it opens up a new audience puts Twitch way ahead for being the better one to stream on. Think of every new viewer as someone who will share your content. You can link your Youtube too and self promote it. There is also far more money on Twitch if you end up getting big at it and far less rules. They are far less picky about the music you have on while doing your thing. You can then edit your video and put it on Youtube or just upload the full live stream to get full effect. It is much more laid back. Also people go on Youtube for edited videos and they go on Twitch for live streaming, you are not really catering to the Youtube audience by streaming on there.
Someone put here it is like promoting on either Facebook or Twitter so just choose whichever one you are bigger on, this is daft, what you do is promote on both.
