Are you buying on-contract or off? If you're buying on-contract with a subsidy, just save up a little more money for the 9700. The 9700 has an amazing keyboard, better than the Curve keyboards. The screen is also a lot better and you can actually use the camera. Plus it seems pretty well built. It's worth the money.
If you can only look at the 9300 and Charm, the 9300 is probably better for messaging. The Charm is going to be limited because of its low-resolution display. Also, I wouldn't expect much battery life out of the Charm with Blur.