I use both TP (Twittelator Pro) and Tweetie extensively. TP is great for badge alerts for DMs, @mentions, shortening URL posted links, and it also auto-refreshes the feed. Lastly, TP has a great search function. Tweetie is great for (as others have said) it's simplicity. I also find it much easier/faster to *follow people on Tweetie than TP. Lastly, if I want to delete a tweet that I already sent, I use Tweetie (which deletes the tweet from all the Twitter servers). TP is my "main Twitter app" but because Tweetie can do 1 or 2 things that TP can't, I keep it around as well.