I actually disagree with the statement that you have to choose either SEO or design.
It is possible to code a site with is SEO, Acessible and design. But what is true is the more spec you add in, the more expensive the product.
SEO is not crucial to get people to your site, its a good thing to include, but you can get people to your site in other ways.
Acessibilty is something somewhat more important then SEO, that is to say if you get people to your site and they can not use it, then the SEO hard work has been wasted. But again its about target market. If its not a legal requirement and your target audience do not need the site to be acessible then its not really that important.
Design is really important, like in real life first impressions count and on a website this is paramount. If the site loads slowly, has spelling mistakes or just looks wrong people wont have trust in you and people will not use your service.
If you take a look at my site
http://www.websitesbymark.co.uk you will see I have used clean visuals and a very simple message. I offer good quality websites at a good value price. For my search terms it lists first page on google, and I use some other techniques for getting customers.
When putting the site together I concentrated on the visuals, then worked on the SEO as a second part to it. I have not gone 100% seo as it will spoil my vision and will not look as good.