Hard question and I went myself through this process two weeks ago.
At the end I had Wordpress and Tumblr on the shortlist and went for Tumblr finally. At the end it heavily depends on your requirements. If want to self host and are heavily into SEO and customization then rather Wordpress, if you want built-in virality, a huge community and a social network touch then Tumblr is your friend. Both are quite powerful but I liked Tumblr a bit more because it feels/looks fresher and I needed it for a private blog and not for creating tons of traffic. Then I'd go for Wordpress with a custom domain and selfhosted.