In my opinion, I would honestly go with the nature: Jolly
Adamant boosts attack, but lowers sp. atk. in exchange. This nature would be really good if your more of an attacker rather than a defender. Boosting his attack - higher than usual - would make him incredibly good at physical attacking. For me, I love to focus more on attacking than defending ; but you could be different. I recommend this nature if you're a hardcore attacker - especially since he has a lot of great moves under physical.
Jolly boosts speed, but lowers sp. atk. in exchange. This nature would suit him because you would start to equalize your speed and attack stats much closer now. Dragonite is a fairly fast Pokemon, but this nature would help. Now, you would have a Pokemon with close stats - making him a terrific Pokemon. This nature would suit him a lot - based on your 2 options.
The only downfall I see here is the decrease of "sp. attack". Sure, dragonite majors in attack much more ; but what about moves like Thunder and Hurricane. Dragonite is the only Pokemon that can learn these moves. Plus, giving him rain dance (or having a support Pokemon with rain dance) could make him incredibly strong. But, the lack of sp. attack wouldn't provide as much power. Unless you have a different moveset in mind - either way it's fine.
Your Choice! Hope I helped! :)