Need help with Dragonite

Both of your Dragonite's have great offensive move set. If you have lots of candies and stardust then you should go for iv96 Dragonite of yours and max it out but since Dratini's are quite rare to find in your place thus making it difficult for you to collect candies then you might consider iv78 since it has the best "rank a" offensive move set. Its a Dragonite after all and will always be stronger and has a higher base stats compared to other Pokémon's that has iv100%! :D

But will always be weak even against a lower iv/cp Lapras with Ice Moves!!! hahaha XD

Since both of them have offensive movesets, I would say power up the weaker one so they are both reasonably strong (2000+ CP).

You should power up both IMO. If you don't have the candy to get the 96% one to your level, focus on the lvl 25 one for now. You can always boost the 96% one later on when you get more candies. They are both amazing attackers, I wish I had either one of them. All my dragonites seem to get Steel Wing and Hyper Beam...

They're both great! You should question yourself "i want one damn strong dragonite to take out gyms or two nicely strong to take out gyms (with less efficience) AND defend gyms?" Dont worry if your moveset is not perfect for defense, a high CP dragonite is still a mofo HIGH CP DRAGONITE.