I have been a Twitter addict for over a year and generally use Tweetie2 as my tool on the iPhone. Recently I became aware of Hootsuie.com and was drawn by the statistical capabilities. Combined with RSS to Twitter, Scheduled Tweets and multiple account posting the tool set should be of interest to any serious Twitterer.
Supporting the web app is a paid iPhone app. I specifically wanted scheduled Tweets on the iPhone but was pleased to see the app packed punch in other area too. The UI is different to any other iPhone Twitter client, in many ways the URL browser is far more intuitive, support for ow.ly URL shortening means stats which are available in app is a great feature.
The ui is cusomisable meaning you can drop data rows you aren't interested in, a great feature.
Hootsuite is a great additional tool to Tweetie 2 however I can't say either has everything you would want in one app.