If you are using it mainly for messaging then the Curve 8320 will suffice. The Curve 8900 does offer, in my opinion, significant upgrades over the 8320. Better camera, 4 times the app memory, GPS, better screen resolution, faster processor, newer OS. However, since you are mainly using it for messaging most of that stuff doesn't really come in to play.
You may want to go with the Curve 8900 as it will be more future-proof than the 8320. You will see more OS updates and such for the Curve 8900 in the future.