I just read the article Can Tumblr Topple Twitter by Paul Sawers and poked around Tumblr a bit more this morning. One thing I realized about Tumblr is that it can look quite a lot like Twitter. You can post brief, tweet-like messages and the page display (using one of the default themes) is quite clean and Twitter-like.
Add to this the searching function, the Reblog buttons, and the ability to follow other users, and you basically have Twitter. Hm.
But, there's also the versatility of it being a blog with titles and of course much longer posts, if you wish. Hm.
Looking at the default set of themes on Posterous, there's no doubt the thinking there is that this is purely a blog. All the themes and layouts look blog-like.
Of course there's nothing preventing you from posting short, subject-less posts of, say, 140 characters or less.