It depends on your interests. It’s not a “choose one” situation. Many runners run both.

i mean i used to do the ces way more than mainboard shit cause after a while you get kinda tired of mainboard but imo you’re not really experiencing too much of the game if youre not doing longer cats and doing mainboard stuff can help you with movement & other shit for ces

Do what is fun for you

You can do whatever, but if you want to improve at general movement, I'd say main board categories are really good for that. I'd recommend at least trying any% out, and if you don't like it as much you can stick with Category Extensions.

just run what you want and you like the most well i would recomend starting getting out at least 1 speedrun of Any%, but then just run what you enjoy