In Jainism, both are avoided because they are root vegetables and involve greater violence to living beings.

If you ask which is worse according to Jain practice:

• Onion and garlic are often considered more unsuitable in many Jain traditions because they are also believed to increase passion, heat, and attachment.
• Potato is also strictly avoided as a root vegetable.

So the simplest Jain answer is:

Neither should be eaten; both are against Jain dietary discipline. If one is compared, onion and garlic are often treated as more objectionable in practice, but Jainism does not encourage ranking them for permission—both are to be avoided.