Definitely YouTube. First of all, because YouTube has many and many useful videos, while Tik Tok is more app for fun. Yes, TikTok may have some good channels which are about science, tech etc., but even such kind of channels is affected by short format videos, which simply don't provide opportunity to reveal fully the theme. I read some articles about dopamine which we gain through the scrolling of our feed - it's crazy, modern social media destroys us, it makes people lazier and quick dopamine kills motivation and desire to reach something. So, I am sure, that long useful videos on YouTube are better than memes in Tik Tok.

P.S. I don't mean that you need to quit watching memes or funny videos, but I really believe that people should decrease amount of time that they spend on it. In nowadays I see that short videos negatively affect society. If we speak about new crypto guys, we can see how lazy they are, they want to receive everything fastly, without putting any effort in it, what is wrong and impossible. Because if you want to become good in something, you must be ready to spend long hours, ask questions and step by step go into the direction of your goal. However, loads of people just want to find the button "MONEY" and press it.

P.S.S. And as practice shows, I am not alone who sees this bad effect of short videos on productivity, mental health, career etc.