Go with Dragonite. Base stats make a large jump from 420 to 600 (180) and he gains about the same amount of resistances as he loses. If you are type matching, you will be fine. Dragonite will also learn Inner Focus (unless you have multiscale). Inner focus will give you a better chance to attack on a regular basis. Dragonite can also learn a wide range of moves from fire to steel to ice to electric, while Dragonair is stuck with mostly dragon moves (excluding tms).
Another upside to both of them is that they can learn most of the TMs for lots of choice in them.
Pick Dragonite! And good luck.