both make you good money, but I have to go with apps, coming from someone who does YT for a living.
YT will pay you anywhere from 0.5 - $2 CPM on average. I've seen $11 CPMs during high season only if partnered with a network. So, if you get 1000 "monetizable" views, you will get $2 on avg.
with an app even if you monetize it with ads, you will keep getting returning players that will watch the ads over and over again. Then what about you open an in-app store, you make more money.
What happen when you do both?
Get YT traffic, send it to your app, monetize even more. There are infinite ways to make money from one project.