But which would be more effective? Advertising on a popular and established webcomic which might cost or hiring a freelance webcomic artist to make a specific comic for your website?
Advertising on any targeted website (webcomic, blog, search engine, etc) is a good way to bring potential customers to your site.
Creating your own unique content (like a webcomic) isn't necessarily a "direct" advertising method, but it can be a good way to get people to your website (to view the comic and what comes next). It can be a way to set your site apart from others in the field.
If you aren't drawing the comics yourself, it might be expensive to continue to hire out a graphic designer to create a comic line on a regular basis.
Places like elance.com, ifreelance.com, designoutpost.com, guru.com are some places where you can find freelance artists.