Really depends on the drake, the team comps, the situation of the game, everything.
Cloud drake: ALWAYS go for the tower
Earth drake: debatable. probably go for the tower. If you expect a long game where the enemy can defend for a long time though, an Earth drake will provide a lot more pressure than an earlier tower which was a foregone conclusion would (i.e. a good waveclear champion or two still can't do much to stop you taking bot lane outer turret eventually, so why take it over a drake?)
Fire drake: 50-50 here. If the enemy team outscales you, take this one. If the enemy team will have a better teamfight, probably take this one.
Ocean drake: Currently, take this one. It's OP AF. It's getting nerfed and won't be as worthwhile afterwards, but right now it's insane.
Biggest problem? People expecting an Ocean drake to spawn and getting a cloud drake instead. Lack of clarity FTL. Spend a lot of resources setting up for the best drake, only to get the worst.
In general: I think outer towers are overrated. In most circumstances they're a foregone conclusion. I don't think I've ever seen a game where top or bot outer turret failed to be taken and the game wasn't a stomp. It's very rare, but I do see non-stomps where a team fails to get mid outer. Overall though, outer turrets are pretty easy to take. So dragons can be more important than these unless you really need that gold advantage. Often a good idea to get some damage on the turret then turn to Dragon before it's too late. The dragon, especially non-Cloud drakes, will apply more overall map pressure and overall turrets-taken on average than directly taking one outer turret (and at times even one inner turret). Of course, an inhibitor turret is ALWAYS worth more than any elemental dragon.