When I saw posterous pitch at the Y combinator demo day, I signed up for an account right away and started doing a lot more mobile blogging. Then after a few days, I thought to myself "isn't this a lot like that tumblr thing all my tech friends are using?"
So I talked to them about the differences, and the general response was that Tumblr was more social, so I setup an account on Tumblr as well.
Then because I was too lazy to thouroughly test out both platforms, I just decided I'd send my blog post via email to both services, so I could get comfortable actively using both platform and seeing the results. The posterous account seemed like a great place to point my non-nerdy non-techy friends, and tumblr would be a place where I could keep up on the tech discussions, since thats where the tech audience already set up camp.
As you can see, they look nothing alike. It turned out there was some major differences. Here they are:
1) Tumblr is more social. All my nerdy friends are already using it, so the dashboard of tumblr blogs i follow is already full of interesting content, and conversations for me to add to.
2) Posterous handles full content posts with an image as well as multiple images. Tumblr doesn't do this all that well.
3) Posterous really works without ever really having to go to visit the site. Tumblr tends to be more useful when you go visit the site and use it.
Nice fun comparison. I probably wouldn't use Tumblr to communicate with my non tech friends. Now that the comparison experiment's over I just post whatever I want to each!