I know twitch has the bigger audience (right now), but I would seriously look at streaming on YouTube instead.
You’ll be allowed higher bitrates and resolutions than a twitch partner.
Viewers can select the quality they want on everyone’s stream. You don’t have to achieve “partner” status.
(I believe) All your streams are automatically saved as VODs on your youtube channel and stay there forever.
Just an overall better streaming platform.
Edit: this only applies if you are not using a capture card
The one thing I forgot to mention is the advantage of the much higher bitrate YouTube allows you (8Mbps vs 3.5Mbps).
3.5Mbps is a pretty low bitrate for HD video, so you need to make every bit count. Pure CPU encoding (x264) will be your best option if you want a good looking stream. Unfortunately this method will have the greatest impact on your game’s performance.
YouTube allows over double that, which means you could use GPU accelerated encoding (NVENC), still have a stream that looks better than the 3.5Mbps Twitch stream, and it comes at a MUCH smaller performance hit than x264.