You can find several dialog system for C2 listed in the how do I FAQ under the section "RPG genre".
As far as I'm concerned, I prefer using text objects (more flexible) for everything that is text/dialog. You can use sprites for the GUI elements that help you navigate through those, but for the displaying of the text itself, it will take far less ressources than having to draw sprite for each dialog.
Also you can use web font with the text object to make it look nice.