Early game, choose one after a double kill in Bot lane assuming you have enough champion power and health and mana to get it quickly.

Choosing to do both is a lot more risky because it gives their jungle, Mid, and potentially their Bot lane time to show up (+ a possible Teleport from their Top laner). And their Bot lane will have topped up on sustain and bought items assuming they had enough gold which means they will be at an item advantage over your Bot lane.

If it something juicy like an Infernal or Mountain Drake then opt for the Drake. If it is something less interesting like a Cloud or Ocean Drake then knock off the tower first.

If you opt to take Drake make sure your Mid and your Bot lane shove their waves in their respective lanes so that their opposing laners have to choose whether to deal with the minions at their tower/in their lane or going to contest the drake. The minions also provide you with some vision in that lane as long as the are alive.

As jungler with Smite it is your job to make the call on whether to try and go for the Drake after the tower in such a scenario. You can say no. Type it out in chat and walk away while dropping a ward on it or taking Scuttle Crab if it is up and you have the time in order to prevent the enemy team from a free take away assuming they did rotate to that side of the map and are coming to contest.

Getting your laners a back to buy and resupply after a double kill and objective is a big deal.

I keep coming back to this topic because I myself have screwed it up many times. By high Silver and Gold everyone knows that if you have an advantage you should try to get some objectives, but we don't know enough to not bite off more than we can chew/don't know when the variance on a set of plays becomes too high. Maybe if I write it out enough I'll have it drilled into my skull. XD

Refined off some things PsiGuard mentioned in a comment to an earlier blog post and some thoughts on the topic from SoloRenektonOnly.