I had a Posterous account (here) before Tumblr (here).
I found Posterous frustratingly devoid of features. Yeah, it’s simple because there’s little one can do there.
Tumblr, on the other hand, offers a simple and rich environment.
Right now, I consider Posterous to be Tumblr-lite. They recently added the ability to theme your Posterous blog… like Tumblr has had for a while now. But Tumblr’s ability to integrate with Twitter and Flickr as well as the fact that it’s nigh instantaneous in response to your posts, changes, subscriptions makes it a win for me.
Flip side, I seem to see more of the celebrity names on Posterous than Tumblr (like Rainn Wilson, Guy Kawasaki, iJustine, etc.)
As far as content goes, well, that’s your decision whichever platform you go with. Blaming the platform for people’s choice of content seems a poor way to decide.
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And I agree that Tumblarity (and its completely unknown algorithm wrapped in a question inside an enigma with secret sauce) is a waste of effort. Just don’t pay attention to it.