The way you charge the battery doesn't affect the way it drains on a day-to-day basis. Fully charged is fully charged. Power is expended by the work that's done.
This takes me back to Elementary Electronics - the water analogy. Say you have a 10-gallon/40 liter water tank. You can fill it with a fire hose in about one second, or you can fill it with a one-cup/250 ML measuring scoop in 10 minutes. But once it's filled, it's filled.
Now, say there's a spigot on the side of the tank. You can open the spigot just a little and it dribbles out slowly - tank is empty in an hour, or you can open it all the way and it gushes out and the tank empties in one minute. Either way, you have 10 gallons of water on the floor.
In other words, the speed of filling the tank has no relation at all to the speed of emptying the tank.
What may matter with the charger is the long-term life of the battery - faster charging (higher wattage charger) may wear out the battery sooner than a slower charger. But no matter what, eventually that battery will wear out. In the case of a decently-designed piece of electronics all the important charging circuitry is built-in (iPhone in this case). The iPhone regulates the power being fed to the battery so that the battery is not exposed to a too-high voltage or charging current.
The thing to worry about with a charging brick is overall build quality - that it actually does put out the specified voltage and current, and that the DC signal is reasonably clean. It's not exactly rocket science and they're not exactly expensive things to build well, but we can't assume everyone is going to bother to build them well.
