Asking whether design or user experience is more important than search engine marketing is a bit like asking if you'd prefer to have the first key you find - or the one that opens the door. The answer is not simple because the two goals of SEO and optimized design for the user are equally key, but accomplish different goals. Unfortunately, those different goals often come in direct conflict with one another.
An example of one SEO trick is to repeat certain keywords on your home page, as the frequency of occurence of keywords can give you a higher ranking in search engines like Google. But, too much repetition and redundancy of keywords on your home page may not be good for the UX once visitors arrive. Keep in mind, good content management systems like Umbraco (for publishing) or Magento for e-commerce will have strong tools for behind the scenes (like meta tagging) that will help keep messy stuff in the background.
But honestly, trying to reach the Google Nirvana is a bit like looking for the pot of gold at the end of a rainbow. Changes to Google's March algorithm have many people scratching their heads. The main sites hurt seem to be content farms like Mahalo, Ezinearaticles.com and Yahoo's Associated Content (which critics say amass content for the sole purpose of luring in search-engine traffic.) Google-generated traffic to each dropped more than 75%, according to software firm Sistrix.
But anyone can get hit: Max Spankie, who operates customer review website My3cents.com, said his site lost a significant portion of its traffic and revenues overnight following Google's algorithm change. That came as quite a surprise to Spankie, since My3cents.com was recently recognized as the top consumer complaint site by the non-profit Consumer Federation of America. Unlike some of the sites that now top My3cents.com in Google's rankings, all complaints on Spankie's site are moderated. Hundreds of companies use the site to interact with customers.
The bottom line is that there is no correct answer and your site needs to balance the two goals of SEO and optimized design. I am not sure there is any methodical, structured system for making these trade offs but collaboration and cooperation between SEO and UX experts is key, and keeping one's eye on the overall business goals is too. In order to help you make some decisions, you can define User Experience as a marketing strategy and SEO as a tool to achieve the strategy (e.g. sell low price over quality).
One final thought: both are essential. Balancing user experience with business needs is what good interaction design should do. SEO done right can help drive traffic to your site, and good user experience will keep that traffic engaged and can lead to more conversions (if you're selling or promoting something). Most importantly, stay true to your brand. Generate good content with frequent updates, strong content hierarchy, and organization. Pair that with solid meta tagging to achieve both SEO as well as well as user experience. This should form the backbone of your site.