Just another note about Posterous. I logged in there recently and noticed that I'd had a great increase in views of the blog I have there. I haven't done any promotion of that particular blog so I can only assume that the site itself is getting a lot more visitors of late.
The traffic I've had there is much greater than at Amplify (although that's respectable). And my Amplify site gets many more hits than my Tumblr blog. So if it's immediate and easy traffic that you're after, then I'd say that Posterous is definitely the go.