How can I use posterous with twitter?

I have hear alot about posterous replacing twitpic as a twitter photo client. Also that is can become sort of a stored blog of your twitter posts. I’ve also seen some people using both tumblr and posterous. I am a tumblr user and am trying to figure out if there is any point in using posterous as well. You can post to twitter from tumblr as well..or am I missing something that differentiates tumblr and posterous? can someone help me? Thanks!