Unequivocally, SEO comes first just like a business plan should come before opening your doors to the public. An SEO/SEM strategy comes first before the website design stage since you need to clearly need to define your audience as well as ways to reach them before you design the tool (your website) to reach them.
Keep in mind that most website designer as skilled as they may be are either oblivious to SEO or simply have no clue about the concept which can cost you dearly if your site is search engine unfriendly and therefore invisible.
