I will list them in order in which i believe you should consider them
1: AT&T Quickfire - excellent phone, BEST keyboard, touch screen can get a little unresponsive at times, HOWEVER, AT&T did recall every Quickfire phone and pulled them off the shelves to cease sales for the time being. Apparently, they wanted to fix some of the issues that the phone was having, so if you want this phone, you'll have to wait a bit until they re-release it or come out with a new version
2: Pantech Matrix - my friend has this phone, and he loves it, sliding design is pretty good, keyboard is nice but a little bit on the smaller side, phone is pretty thick however, and I've heard that many people have trouble with it breaking, as long as you're careful, you should have a great time with this phone.
3: Pantech Slate - the reason i put this one third is solely because of the keyboard, besides the Quickfire, it has the best keyboard of all the new messaging phones, and it's quite thin unlike it's Pantech counterpart, the Matrix, The only problem with this phone is that it lacks 3G data support and it has no slot for micro SD expansion cards (important for more pictures and music on your phone).
4: Samsung Propel - the reason i put this last is solely because of design, the keyboard is very flat as well as very small, which provides a bit of trouble for really quick texting, the phone's overall design seems lacking (it's probably the bigger of the phones, but it has the least amount of features, whereas the Slate lacks features, it is much thinner), i feel that Samsung could've added more to make this phone better, nonetheless, it still proves to be a great phone
The choice between these phones really isn't that monumental, because they are all great phones and I'm sure you'd enjoy any of the four that you purchase.