All other things being equal, the only difference is the current-carrying ability. Better depends on other things not being equal.
In addition to current-carrying ability, 15 amp power cables have a different female end on them than a 20 amp power cable.
These are not compatable with each other. Therefore, you must know which is the correct one for your gear.
A good majority of the equipment out there use a 15 amp IEC connection. 20 amp are more common on large amplifiers, etc.