You're right that Tumblr's social aspects are more developed than Posterous's and that this has been a huge driver of their growth but to me that's not so much solving two different problems as Tumblr being more smartly designed -- realizing that blogging, even minimalist blogging, is inherently social.
As for Tumblr's top tier investors -- sure, that's true. I was trying to emphasize that Posterous, on paper, has everything it takes to win.
I chose Tumblr over Posterous because of the elegant templates and the art/music tastes, so I agree here. If Posterous compares itself to Gmail for blogging, then Tumblr is like a public Evernote for scrapbooking. I'm not using it to promote my own ideas or build my professional "brand". Posterous seems as though it's trying to cater both to the computer illiterate and the time-crunched techie/professional who lives and dies by email to organize life a la GTD. For example, Tumblr has a quotations format separate from the text post option, more useful to me than the Google analytics tool that Posterous offers.
The downside is that Tumblr does have a greater share of Livejournal/teen angst, but I've carved out my niche of grad students, graphic designers, and classical music aficionados who upload great performances. The fact that TED Chris and John Maeda (President of RISD) use Posterous is tempting, but also makes it more businesslike.
I have yet to understand why I would want to email posts other than videos, as it's not particularly onerous to use bookmarklets. And Posting within Tumblr means my tags autocomplete, whereas Posterous has you write tags in your email subject line. Posting videos to Posterous is easier though, as Tumblr requires videos first to be hosted on Vimeo. Posterous might have another advantage as it is more useful to coordinate updates to a multitude of other sites, including Tumblr. But I screen different content on Facebook, Twitter, Wordpress, and Tumblr, so I actually don't want the same things posting on all my sites.
As for Tumblr's top tier investors -- sure, that's true. I was trying to emphasize that Posterous, on paper, has everything it takes to win.