What is a T-Mobile phone that is good for texting?

[izzy =)]

Im getting a new phone for tmobile (I already have the Samsung Gravity) and I dont know which one to get. The price doesnt really matter too much, all that I really want in the phone is that it saves texts as conversations instead of single text messages. (It would make the screen look something like this) http://farm4.static.flickr.com/3097/2685527334_25b4eb8908_o.png