This is actually so tricky. My instinct said “right hand side would be impractical, so no way” but then I remembered they evolved long necks to get to food so it makes sense to evolve arms there to better feed themselves.

My little cope against this is:

1. Centaurs use their arms to wield and craft weapons. This would make them much better suited for the left hand option, where they can try for body strikes against centaurs rather than just decapitation. It would also make them actually useful against any foes which aren’t birds.

2. From an evolutionary and physiological standpoint, arms growing out of your neck is unrealistic. You don’t have the structural components to support it there - the neck is too busy supporting the head and it’s pretty crowded up there. Arms are typically attached to a broadened torso-skeleton, built to support lateral attachments as well as the longitudinal attachment of a head and neck.