Hello my friend, It is very true said that both of them are important but the priority depends on the current need of your business. If at current moment you are not on the top of the pages but it is very important for you to reach the people then e-mail marketing is really important and is the priority. In some other situation when you don't have enough spam-free e-mail ids SEO will be the priority so that you could collect some e-mail ids through signup or opt-ins through your site.