I am looking to do some Christmas video content, which mainly involves building trees, lights and decorations in Minecraft and just festive gaming basically. What would be better to do it on? Twitch or YouTube?

Twitch if you want to do it live.
YouTube if you don't care about having a live audience and just want to upload finished thingies to link to/share.

Well...Twitch is for doing live streams and YouTube is for prerecorded videos. Which do you want to do?

With Twitch, it'll probably be a relatively small crowd of people during the stream only. With YouTube, the video is on equal footing as most other new videos, so it could continue to amass views for years.

Yes. I know Twitch is live, but doesn't it store live-streams as videos to watch later anyway? What is also harder to do - produce a YouTube video or live-stream on Twitch?

YouTube.

I think you have to set it up to store them permanently (on Twitch). Otherwise they may cycle (limit to the "log") and be lost. But I'm not sure about that.