Most iPad application designers, I suspect, are suffering from the same mental block.
When I look at TweetDeck for the iPad I feel like the designers have not really thought about the new medium and its capabilities. I am pretty certain that some one just decided to do an iPad version of TweetDeck to capture the market. Instead of appreciating the fact that there is an iPad version, I lost some respect for the designers at TweetDeck after looking at the iPad version.
On the other hand, the designers of the twitter official iPad app have done a good job of building for the iPad. So I picked videos of users using both the apps and posted them here so that you can see the apps and make a call yourself. These are not marketing videos by the firms.
Twitter for iPad video by a user
You'll note that the above user also had other twitter apps on her desktop. She uses the words, devotee, love and awesome while she talks about Twitter for iPad.
TweetDeck Video by a User
Don't do the mistake that the TweetDeck guys did. When you design for the iPad, start from scratch and build it for the iPad.