Short answer: In Jain Dharma, “better” depends on your bhav (inner purity), not on how long or hard the fast is. Athai is a tougher austerity than ekashana, but a sincere ekashana done with samyak bhav, vinay, and careful maryada can be spiritually superior to an athai done with ego, show, or carelessness.

What each means

• Ekashana: One meal in a day, taken before sunset; the rest of the day is restraint, svadhyaya, samayik, pratikraman, and only boiled water within maryada.
• Athai (Atthai): Eight continuous days of upvās. Traditionally no food; water rules follow your maryada. It is an advanced tapa and should be done only with guidance and fitness.

How to think about “which is better”

• Scale of austerity: Athai is heavier tapas and, when done with pure bhav, supports stronger nirjarā (shedding of karma).
• Scale of purity: Any tapa—small or great—yields real benefit only with right faith, right conduct, and control over krodh, mān, māyā, lobh. A calm, humble ekashana can bind less pāp and more puṇya than a prideful athai.
• Ahimsā to oneself: Jainism never approves self‑harm. Choose the vrata that maintains health, stability, and steady sādhanā.

Digambar–Shvetāmbara notes

• Both honor ekashana and athai. Shvetāmbara often link “aathai” with the 8 days of Paryushan; Digambar observe Das‑Lakshan for 10 days, while an 8‑day fast (Atthai Tap) is also respected.
• Water maryada and exact food rules differ by tradition and guru; follow your sangh/ācārya’s instructions.

Practical guidance

• Take niyam with a Guru/elder. Build gradually: ayambil → ekashana → upvās → attham/athai.
• Keep daily pratikraman, alochanā, svadhyāy, and kāyotsarg; avoid show and comparisons.
• Do proper pārṇā as per maryada. If unwell, shift to a lighter vrata without guilt; purity of intent matters most.

Essence Neither ekashana nor athai is “automatically better.” The best tapa for you is the one you can do steadily, safely, and humbly, with right faith and right conduct—leading to real samvar and nirjarā.