First of all, Tumblr is comparable WordPress.com, but not WordPress.org. WordPress offers 2 services. Tumblr is a private company with millions of dollars invested in it while WordPress is open source. The majority of Tumblr users are homosexual, female and in their mid 20s or younger. The majority or WordPress users are men in their mid 20s and older. Most people use Tumblr to entertainment themselves. Most WordPress user do so to generate ad click profit and website traffic.
If you want to make money or generate web traffic, you will want to learn how to use WordPress. If you want to make new friends and meet young people who dabble in blogging and like to take photos for fun, you would join Tumblr.
WordPress.org one can download the WordPress application and install it to their own server allowing them to use a plethora of themes and plugins no other blog can compare to. You can also manipulate the code of the installed WordPress (self hosted) if you have programming skills.
WordPress.com is very similar to Tumblr. It's a free subdomain blogging service where themes and options are limited. Users with accounts on WordPress.com can search each other's blog conveniently and contact one another similar to Tumblr. I do think that Tumblr is slightly better than WordPress.com.
What Tumblr can offer you that WordPress.com can't
Tumblr is a lot easier to use the WordPress. Tumblr has a simpler minimal feel more so than WordPress.com while still maintaining a comparable aesthetic web 2.0 feel. Tumblr is a trend right now, and although it was spurred by money, the blogging community is more active then WordPress.com's community none-the-less. There is also a current image posting trend called Instagram that Tumblr has contracted with so that Instagram users can easily post their daily pics on a Tumblr post from their smart phones. Tumblr entertainingly keeps their community active. I could be wrong about this.. there's actually word that Tumblr might be a bot fest.
What WordPress.org can offer you that Tumblr can't
WordPress can offer you easy AdSense and email list integration along with many more standard plugins. WordPress has been around much longer then Tumblr and so WP has many more options. The self hosted application WordPress.org gives you no limitations on how you can modify scripts unlike Tumblr. The fact that WordPress is open-source means there's many more developer's contributing to WordPress so you have many more themes plugins and addons to choose from. WordPress (self hosted) is slightly more SEO friendly then Tumblr.
If you want to blog for fun, Tumblr is you blog. If you want to make money from your blog, WordPress is what you want
If you are looking to create a blog you might consider the Affordable Website Package. Included is a website, web hosting, a logo, email list, SEO and support. It offers the best products and methods for blogging.