I will give you the pros and cons of each platform. Tumbler is easy to use, great for displaying images, artwork etc, has a large community to share, re-blog, get traffic and its free. But not a good platform for text heavy blogs, has a largely younger audience, it's strictly for blogs and cant be turned up as a fully functioning full fledged website.
Wordpress.com(Free) on the other hand is a great blogging platform and has tons and tons of functionality. It has the most options for design but you cant use plugins with this. The Wordpress.org(Paid) has all the same features as the free version plus more and you can turn it as a full fledged website easily. Cons, you will have to pay for it.
Hope this helps.