Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Twitterfon Pro is rumored to have push on the way, so I'd go for that one out of the two you mentioned. I can't stand the dull gray icon though.

Personally, I think Tweetie is the best one out there.
 
I swear by TwitterFon Pro. Swear by it.

However, I am eagerly awaiting Steven Troughton-Smith's "Chalk" app to be released, from screenshots he's been posting to TwitPic it looks so damned good!!
 
Tweetie for me but it has some glaring bugs under OS 3.0 at the moment. Apple are being very slow in approving the updated build. Or you could try TweetDeck which syncs with the desktop version very nicely.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.