SEO because you can learn it yourself and it costs nothing. Adwords is a very quick way to drive targeted traffic.
Check out this article: http://www.jwrmedia.com/advertising_strategies/which-is-better-ppc-or-seo/ PPC gets you on page 1 instantly, but if you are in a competitive market you will find it hard to pull a good profit if you use PPC as your primary advertising source. SEO will pay off much more in the long run as you will rank well organically for many terms/keywords and cost much, much less over the course of months or years.
PPC can be very expensive these days in any niche with decent competition. Sticking with long-tail keywords and studying a good PPC guide so you know what you're doing would be well advised before opening your bank balance to PPC! Great for testing keywords & ads with quick traffic, but very difficult to profit from. Plus it won't just be potential customers clicking on your ads and costing you money. SEO is a must for the long term. Structure your site right around your keywords with quality, original content and get lots of related backlinks to your site, for example by spinning your site content and submitting as articles to article directories. Takes time but it's free. Don't forget Web2.0 methods for free traffic too...forums, video sites, social networking sites, groups, bookmarking etc. Huge potential there.