In Jain Dharma, both potato and onion are root vegetables (kand‑mool). Uprooting them kills the whole plant and disturbs countless tiny lives in the soil (anant‑kay jivas). So, from the view of ahimsa, both are to be avoided and are equally himsakar.

• Monks/nuns: strictly avoid all root vegetables, including potato and onion.
• Householders: are advised to avoid both. In many traditions, onion/garlic are kept under even stricter avoidance because they disturb mental calm and provoke passions (pramāda/kashāy). But the main reason for all roots—including potato—is himsa.

Essence: Do not rank them. For a Jain following ahimsa, both potato and onion are unsuitable; best is to avoid both.

More on why Jains avoid roots and specifically onion/potato: