So, I've somewhat played with both today, as I've always have been looking for that perfect clipboard/status update thingy, kind of like a notepad/del.icio.us/whatever-else hybrid. I got the urge of trying out Twitter yesterday, first just the web interface on Linux, then today got to use Twitterrific on Mac OS X (which is absolutely awesome). Well, the web interface for Twitter certainly isn't as polished as I'd like it to be, but that's not where Twitter shines. It's all the other possible interfaces. There is an API for people to write 3rd party apps (like Twitterrific, and many other ones for Windows, Mac OS X, Linux, and web browsers), IM integration (where you can message one of Twitter's screennames (like firstname.lastname@example.org for Jabber/GTalk)), and last but not least, SMS messaging from cell phones. I think I'm going to continue using Twitter for a bit, and see how it goes. But up to now, I pretty much like it.
Now, on to Pownce. Apparently it is just recently rolled out, and is still an invite-only beta of sorts. It's got a much better web interface, and can let you not just post text, but also share links, files, events, and such with your friends. But, compared to Twitter, Pownce doesn't have the SMS feature, nor an open API for other developers to play with. But hey, Pownce is still very new, so perhaps I should give it a little more time. I do like the extra things Pownce brings, but it still needs time to grow.
Also, something I didn't mention. Both these sites have some social networking involved also (you see friend's updates, etc.) and I can see this being a better way of keeping track of what's happening than the status updates of Facebook that no one really uses these days.
P.S. If anyone wants a Pownce invite, I still have a few left. Just message me or something. Also, my username/id on both sites is LokiSnake, add me if you want, as I'm still kind of lonely on those two sites.