The output voltage will always be 5 volts. The amperage determines how fast it charges. More amps the faster it charges. A standard PC USB port generally will put somewhere along the lines of 0.5 amp, sometimes a little less (if you actually measure it).
I've found on all my devices my batteries last longer on each charge if I slow charge them. So I purposely slow charge them whenever possible. I'm typing this right now from a OnePlus device with a 4.0 amp charger that has basically never been used. While most will say Li-Ion batteries don't have a memory, or will be affected by the fast charge, every time I've tried testing slow vs fast charging slow always lasts longer.
Input doesn't really matter. Input voltage needs to match your countries voltage. However many chargers will say 110-220 as they will work on either voltage. As for the amperage, if you are looking are two chargers and they both put out the same current on the output, but one requires more input own of two things is happening:
1: One is a lower quality, and is using more energy to do the same thing.
2: One of them is lying, and is not outputting what it claims.
Either way, generally Anker is quality stuff. Definitely stick to a quality brand, an OEM charger or something known like Anker. Don't let my affinity for slow charging scare you away from fast charging either. Just maybe give it a whirl, try charging your phone from a PC USB port and see if your phone last any longer on that charge. It may not for you.