A list of pros and cons of Twitter and Tumblr now that I've tested both sites (it always takes me a while to get accustomed to using a site, and I usually leave after a few weeks):
They are both cluttered with suggestions of whose Tumblr/Twitter one should follow, and Tumblr is worse (and uglier!) than Twitter in this aspect. One dreadful way of wasting time is skimming through the long random list of suggestions Tumblr has dumped on you to find the posts of the people you actually follow.
Tumblr lets you edit the entries while Twitter only lets you delete them, which results in broken links. Since I make typos in almost every sentence or leave out words or even groups of words whenever I type fast, the lack of an edit-button irks me. It's different on paper because I seldom misspell when I write, but typing (especially typing on the phone) is a different sort of beast! (I'm convinced that the screen effectively shuts down a part of my brain.)
It's easy to like and link and reblog artworks on Tumblr, but Tumblr generally doesn't foster verbal communication. Fandom tends to share links to Japanese fanart (originally found on Pixiv) on Tumblr so that I'm going to keep my account there.
Discussions on Twitter are a pain since you can only write one or two sentences until you have to tweet anew (and spam the mailbox of the person you're trying to talk to).
It's easy to rant about your day and post random observations and trivia on Twitter, which is why Twitter is good for short announcements and exclamations or for saving notes on your stream of consciousness, which you can use for longer journal entries later. It's also easy to post (fan)art WIPs directly from Autodesk Sketchbook to Twitter (although you might as well post the WIPs directly to Deviantart). But since I tend to waste time tweeting random thoughts which I might as well keep to myself or jot down into my paper notebook, I'm only going to use Twitter to post fic/art update related announcements from now on.
All in all, I like Dreamwidth so much more than Twitter and Tumblr and wish I would finally solve my login problems on Dreamwidth. I can only use Dreamwidth on the laptop after resetting my password and activating the "Remember me" box; and I might have to reset my password again after the next computer update.