From Compete - yearly change:

tumblr: +184.36%

posterous: +297.36%

So, uh, even with the last 3 months in posterous is still growing faster than tumblr. If it were a stock its valuation would have increased more than tumblr. Also, wouldn't there definitely be less hits to posterous based on the fact that you EMAIL posts in rather than sign in? That being said, the tumblr landing experience is probably one of the reasons I blog with them and not posterous.