Tweetie vs Tweetdeck
I've been using Tweetdeck for quite some time. I really like the sleek columns that allow for easy filters on keywords. Having real-time updates on those filters is useful if you are trying track a single subject like "Drupal" or "pingV". The filters are saved when you close down Tweetdeck which is handy because the Adobe Air application appears to have memory leaks.
Tweetie allows you to have your friend stream, replies, direct message, and one search in a single column. By clicking twice on a tweet which has replies, it gives you a view of all messages in that thread. You can create new searches that run parallel to the primary window. However, when you shut Tweetie down, all your searches are lost and you have to recreate them from scratch. Tweetie is visually beautiful.
The Tweetie interface is intuitive and easy to use. However, unlike Tweetdeck, it doesn't seem to have an audio or visual queue that a new Tweet has been added to your stream. That could be good or bad depending on how you are using Twitter.
Tweetie has both a paid and a nonpaid license. If you choose to not pay, the software is supported by ads.
Oh, and finally, Tweetie is ONLY for the Mac.