You can't host your own Posterous and it has fewer features and no third party plugins but it's easier to use, has built in syndication to TwitFace, and is probably far more secure than WordPress.
(Edit: How is this wrong? Was it my playful nickname for Twitter and Facebook collectively?)
I think the Posterous/Tumblr dichotomy is a false one. Where they're both ultra-simple microblogging platforms, tumblr is easily more community centric, insular in focus than Posterous, and in my opinion that's where it's strength is.
Nearly every avenue of a default feature now found in tumblr has come from innovation that users have made use of when the site was still young and lacked considerable features.
The difference is obvious: Posterous lets you post anything from anywhere, and they host it for you with zero setup. Of course the key feature for its success is email posting!
It's funny because email is how I started using Posterous. I thought the getting started process was clever. Over time, I started using the bookmarklet more and more. After a year, I would say I use the email feature extremely rarely.
Plus the easy reposting to so many other services. I really like that Posterous can repost to not just Twitter, but Identi.ca, FriendFeed, and Jaiku. And Wordpress, Blogger, Tumblr, and every other blog site. And Youtube, Vimeo, Blip.tv, Flickr and every other media site.
Etc. etc... The autopost feature + post by email is the dog's bollocks.
One has an elite startup feel, and the other is a platform for "normal" people that just happens to have a popular open source version that you can install on your own server.
