Well I think several questions need to be asked to decide "Tweetcaster or Tweetdeck"?
1. Do you use Facebook?
2. Do you wanna be able to see Facebook statuses also and not just post status.
3. If you don't use Facebook which UI do you like better?
If you use separate app for Facebook it's your phone. Don't ask input. It's your phone use the one you want.
Personally, I use Tweetdeck because I use both Twitter and Facebook and I wanna see status updates from Facebook and tweets from Twitter as well as wanna be able to post to both. Tweetdeck also has Google Buzz and Foursquare if you use those protocols so you won't need all these multiple apps.
Do remember it's your phone and your choice.