I have been trying out Posterous and Tumblr recently. While Tumblr is a very elegant tool, with great designs, the blogs do seem to take a while to get indexed by Google for some reason.
Posterous blogs are not nearly as pretty, and when you log in the interface is a bit more clunky. However, it's the clear winner when it comes to SEO, in my opinion. I have only recently started a blog there and it is already fully indexed.
Posterous also seems to get a lot more traffic from other users. There is a basic counter that records page views in the dashboard. Even though I've been doing very little link building, it's still racking up more and more hits. I've installed a counter on my Tumblr blog and I haven't had much traffic at all.
That said, Tumblr is still a good tool - particularly if you value aesthetics highly in your blogging.