The choice usually comes down to your available time, the complexity of your site, and how quickly you genuinely need results, not a simple better-or-worse comparison.
Consider Your Site's Complexity Honestly
A simple site with a handful of pages is realistic to manage yourself; a large, technically complex site often benefits from specialized expertise sooner.
Weigh Your Time Against the Cost of Outside Help
If your time is better spent elsewhere in the business, paying for help may be more efficient even at an early stage.
Start DIY, and Reassess as You Grow
Many businesses reasonably begin with a DIY approach and bring in outside expertise once the scale of the work outpaces what they can manage alone.
Next step: Use the SEO Audit to get an objective read on your site's current issues before deciding whether to tackle them yourself.