BOTH is important. You need to find sites that will put your link on them that are highly ranked themselves, which will help you, and have high traffic, which will hopefully send people to you. So you both raise your SEO rankings and hopefully increase your actual traffic to your site. So both are equally important to me.
Both are really importent. If the site is relevent and has no traffic then you wont gain any traffic from it. If the site has traffic but isnt relevent then no one will click your link. If youre just looking to build SERPs then go for the relevent site.