Aside from learning about the meme that New York startup have better design, I cannot agree with the author.
I use both. I think they are different services. That doesn’t mean that they don’t compete, but it means that you can use both.
Tumblr = social network/mini-blog
Posterous = easiest way to push content into multiple sources via email (and also the rich text bookmarklet is great, if a little problematic.)
Personally, I think these two are eating up all the people that would’ve gone to Blogger (which I also have). They are both direct inheritors of the whole “push-button blog” idea.