I think this is actually an unfair comparison. For one thing Tumblr really works the social network aspect of the site (reblogging, hearting articles, following users) whereas posterous is more an "island" blog.<p>Which is fine, as they both solve different problems.<p>I also think Tumblr is a technically superior product as well as a superior designed product (right now): for example I can embed javascript into my Tumblr, posterous doesn't allow that. Tumblr has fantastic iPhone and mobile applications, and generally feels, well, more mature.<p>You mention Posterous have top tier investors and developers, Tumblr does too. Tumblr has spark capital and union square ventures as their investors. Marco Arment, the technical lead at Tumblr is a perfect balance of technical chops, getting stuff done and doing it right.