As long as you don't plan on using extensive scaling, this is a moot point. People always obsess about using vectors when they actually don't need them and of course many times AE's limitations will often nullify any benefits vectors may have like effects enforcing rasterization, anyway. In the end it's one of those "it depends" things and you should decide on an "as needed" basis rather than on a generalized level.
This depends on the style you want and your skill level. I do better at cartoons with Illustrator but I do better at realistic painting with Photoshop. If you are good enough you can do amazing things with Illustrator. Take a look at some of the illustrations by Paul Brush.
The only think you have to pay attention to if you create in Photoshop is image size. You want to make your illustrations big enough that you never have to scale them over 100% when you animate. There's no such restriction with vector art. You can scale it to any size you want.
Yeah, I think I'll go with bitmap on photoshop because in the end result I'll be exporting a 2160p video and uploading it to youtube. I am much more experienced with photoshop compared to illustrator and the reason I was thinking about using vectors is for the re scaling abilities it allows.
Yeah, the main reason I was thinking about using illustrator is the fact it uses points and not pixels.
... exporting a 2160p video,,,
Are you sure you want to do YouTube 4K? It's true that YouTube is supporting that right now and I'm all for going with the highest quality that your system can handle, but most folks and devices won't be able to playback 4K for a while. Rendering time for 4K can be as much as 4 times higher than rendering for HD. `Here's another animation hint. If you like the look of classic cartoons like Bugs Bunny then you should know that they photographed each animation cell twice so the movie was really 12 fps, not 24. If I were making classic cartoon animations and especially in YouTube 4K I would set the frame rate for my comp at 12 or 15 and then a production master using the Render Cue, then use the production master to render the 24 or 30 fps h.264 with the AME. At least I would do some testing to help manage the 4K rendering times.