in a one-on-one fight, the Dragoon would win, armor, jump, stronger weapon, the red mage would have a pretty hard time.

I don't have a personal preference though, it's all about what kind of party you're trying to have. if you need another strong melee character, and want some diversity, i pick a dragoon, but if I need to balance a party out, I go with a Red Mage.

Or in IX, get both, and use Freya.