The key difference is who does the work.
With an AI builder, you save budget upfront — but you write the prompts, evaluate the options, make the revisions, and handle the forms and analytics yourself. You spend less money, but more time and attention.
With an agency, you are paying not just for the build, but for the team taking on most of the process: defining the goal, structuring the site, writing copy, designing it, handling the technical side, and delivering a working result. You still need to be involved and make decisions — but the volume of manual work and uncertainty is considerably lower.