You shouldn't buy neither A nor B
A mobile charger with 5V input should have an amperage of 1A or more for fast charging its own battery.
With only 750mA input current the charging of the charger's battery will take its time.
Another item, with your discription, the charger's battery capacity isn't shown (I take it, this 750mAh is a slip of the pen, it means 750mA input current).
Mobile charger' battery capacity should be at least twice as large as the phone's battery capacity.
To be sure, use a mobile charger like the Anker 5600mAh.
Input 5V/1A, Output 5V/1A,
With its microUSB adapter cable it'll charge your phone fast with up to 1A