There has been a lot of debate pinning Posterous against Tumblr. For a little frame of reference, I'm posting this on Posterous...but it will then be auto-posted to my Tumblr feed. I don't believe I can get my Tumblr posts auto-transferred to Posterous. This post could also be auto-tweeted and pushed to basically any other platform out there. If I am not mistaken, Tumblr can auto-post to Twitter and Twitter alone.
I use Tumblr far more than I use Posterous because I like the simplicity and design abilities better. I can direct somebody to my Tumblr page and they immediately know it is me without digging into any of the content. I basically started my journey into social media on Tumblr, so my audience is slightly larger on Tumblr (emphasis on slightly).
While it is a nice thought to allow users to post anytime and anywhere via email, I feel like posting to Posterous is kind of clunky. If the main draw is posting via email, Tumblr also has that option and then when I go to the website, posting is simpler.
I don't subscribe to many Posterous feeds, but the few I do subscribe to confuse me a bit. My subscriptions are laid out very ambiguously and they are difficult to keep track of.
Many of the differences between Tumblr and Posterous come down to preference, and I just happen to prefer Tumblr. I will continue to use both services and see if I can fall in love with Posterous as many of already have.