I like the casual approach of both, but Twitter requires more work than I'm really willing to give such a tiny thing. I like how it works as a chat channel via IM though even with only Scoble and Chris on there it got too noisy for me to handle. Google Talk status messages work fine as a "what's up" kind of thing and I don't have to deal with a whole new friend network.
Tumblr is fun for casual blog crap I wouldn't put here or in del.icio.us or over on In4mador!. Adding stuff to it is a multiple page procedure though (their bookmarklet doesn't do chats or quotes) and I'm not quite sure what to do with it or why I'd keep at posting to it.