When you start a new business, I assume you have little money.
In that case, I would pick:
Content marketing
because...
– at the start, you have time > money
– the advertisement from the very beginning can feel "aggressive"
– people should first get to know what you do (who you are) before you immediately push your offer on them.
When you have money, you can add performance marketing (ads).
Use ideally both of them.
What would be your start?