I recently ran a bunch of tests and confirmed that the iPhone will draw the same exact amount of power on either the iPhone or iPad USB charging adapters.
The iPhone limits it's intake to 1 amp, so using an iPad charger won't cause any problem.
Since the iPad and iPhone chargers are the same price, I would just get the iPad charger so it'll always work with both unless you're specifically interested in the smaller size.
I also tested about 12 different 3rd party adapters I've collected over the years from 5 watt to 15 watt, and as long as they say they're intended for Apple devices, they all performed the same.on iPhone and iPad.
Technically the iPhone uses 0.93 A rapid charging (battery below 80%) and 1.03 A when the screen is on while rapid charging on both iPad and iPhone chargers. It appears to pull more current to power the device so that it doesn't reduce the current to the charger (so turning off the phone shouldn't make it charge any faster.) Above 80% the iPhone and iPad both gradually reduce charging speed as it gets closer to 100%, and the decline is almost linear above 85% (but not exactly.)
FYI: The Amazon 5W USB charger also performs identically to the 5W Apple iPhone charger.
Amazon.com: Amazon 5W USB Charger: Kindle Store
I've picked up a few on various lightning deals for $5 each time. They're really compact like the Apple ones.
The AmazonBasics 2.1 A charger works great too, but the size and shape isn't as ideal.
There's an Amazon Deals app for iPhone. I set up alerts on the electronics category and to catch deals. :yes: