Where you start depends on your budget and your business model.
As a rule of thumb, you can expect quicker results with SEM. So that’s probably where you should start.
There’s a catch, though: SEM costs money.
If you don’t have a lot of money to spend on advertising you’ll likely need to create outstanding content and optimize it for the search engines.
Another caveat: if you do have money, there’s nothing that prevents you from practicing both strategies right out of the gate. That’s how you’ll get the edge over the competition.