If you have trouble deciding between Twitter and Tumblr, this could just make the choice easier. Tumblr has implemented Twitter integration, so you can now both read and update your Twitter status directly from the Tumblr dashboard.
You need to first enable Twitter support via the Tumblr settings page. You can opt to directly send your Tumblr posts to Twitter (like TwitterFeed) or show your Twitter updates on your Tumblr dashboard.
The best feature of this implementation, aside from importing Twitter avatars, expanding short links, and buttons for favorite, reply, reblog actions, is the automatic nesting of tweets, which makes it much easier to follow to and fro conversations.