I think both of them are important.
When it comes to design, you don't need to be perfectionism. As long as your website is user-friendly and easy to navigate, then you won't encounter any problem. Sometimes, simplicity is more.
As for the SEO, it depends on you who will do these stuff in your site. SEO is important so that your website is visible to any search engines after a certain keyword is typed.
It's either you do the job (but it requires your dedication and time) or you let experts to do it for you. Try seodataservice website and see if you can find there what you are looking for.
Basic rule is you conduct first your research and focus on what matters most.