Luckily they are both fairly easy to use and provide many options for templates as well as customization. The tough part is choosing which platform fits your needs and wants for the perfect place to host your vivid content.
In short, Tumblr is free and WordPress is not.
However, if you care less about having your own domain used then you could use the free wordpress.com site instead of wordpress.org. This is definitely not advised as there are less options for customization and limited features.
With WordPress.org the cost is not for the site, but for the hosting, design and development of customizing it.
Note: Tumblr cannot be hosted elsewhere. The blog would be hosted on Tumblr itself.
WordPress is scalable and can be customized with widgets and custom CSS to a full blown out website if desired.
WordPress easily supports widgets and plugins which can be added simply without worrying about hard coding, while Tumblr must be custom coded for similar functionality and plugins.
To read the rest of the differences (and you definitely want to know about SEO) between WordPress and Tumblr (including my bonus tip), check out my original post for Startup Fashion here.