TweetDeck Pros:

• The iPhone app seems to do everything that the desktop app does.
• It has functionality for groups.
• It’s speed is impressive.
• It shows a little API usage counter for those interested.
• I’ts free.

TweetDeck Cons:

• The interface does not feel native to the iPhone.
• It is difficult to navigate around certain aspects of Twitter (some may argue but I dislike how it handles profiles).
• By default it is missing the ‘nearby’ feature that many other Twitter apps are sporting already (I know it can be added via search query, but I would imagine the average user would not know how to do it).
• In order to search you have to add a column (Which I find rather irritating).
• There does not seem to be a way to view trends.
• Shake to Refresh is for morons.

Tweetie Pros:

• The iPhone app has an nearly identical desktop app for the Mac.
• The speed makes most other Twitter apps look bad.
• The interface was made to work perfectly on the iPhone.
• All the features you would expect from a mobile Twitter app (and more) are included (nearby, trends, search).
• Searching for tweets or users is a breeze.
• Profiles are extremely comfortable to view and follow/unfollow.

Tweetie Cons:

• It does not have groups (I personally dont understand the need for groups, but it guess it counts as a missing feature).
• If you’re a massive tweeter/tweet-checker you will have no idea when you’re close to the hourly API limit.
• Version 1.3.1 of the app on firmware 3.0 seems to only display one nearby or search entry at a time (have to press load more to get one more each time – never had a problem before). [EDIT]This was caused by the Twitpocalypse and has been fixed in version 1.3.2[/EDIT]
• There is a one time $3 fee to purchase the app (which I think is fair asking for this app).

In the end this is just my opinion, and some of my statements above may be untrue based on lack of knowledge on my part, but that’s how I see it.

I’ll stick with Tweetie.