Once a visitor reaches your site, is not the good contents always what he (to him the 'type of SA lets assume) will seek there. An attractive design of Web obliges a visitor to remain longer and facilitates it so that it concentrates on the contents without too many distractions.
Thus do SEO and design of Web have their own advantages but which is more important? Without SEO your Web site will obtain only the quantity of the minimum traffic independently of what a great website design of your website A. Moreover, without traffic all the hard labour that you put in the design will not even obtain not appreciated. However, good SEO can obtain much traffic to you but much of this traffic can not remain longer than little of seconds if your Web site has a horrible design.
Thus, I believe that SEO and design of Web are extremely important to return a Web site been successful. However, since I must choose one, I will select SEO. But I do not say that you should only concentrate on SEO and be unaware of the design of Web. Take your time to establish the Web site carefully. Spend the sufficient number of hours on both but spend a little more time on SEO. In this way you will obtain the traffic and will always have a reasonably attractive Web site.