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.
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.