I don't think it's "better" per se but rather more tailored toward a different group of people. Each one has their pros.
The final straw was posting code. Tumblr makes that stupid easy (since they accept Markdown), so I can just do that instead of trying to figure out the magic incantation to make it work on Posterous (gists or something? I don't know. It wasn't documented when I last tried).
Markdown etc has been a much-wanted feature, even for ourselves internally. We try to focus on normals as much as possible, so it hasn't hit the top of our priority list yet, but it has been rising in the list without a doubt.
I will say though that their service is dead simple to use. But then again the Facebook and Flickr exporters are pretty easy to use, too.