Short answer: In Jain Dharma, neither Aayambil (aambil) nor Upvaas (upwas) is declared “more powerful” in an absolute way. The punya depends mainly on bhava (pure intention), proper niyam, and careful conduct. Both are respected forms of bahya-tap (external austerity). However, many Acharyas explain that Aayambil has a unique edge in controlling “rasa” (attachment to taste), which is why people sometimes feel it works more deeply on day-to-day passions.

Clear comparison in simple terms

• What is controlled

- Upvaas: Anashan (no food for the day). It restrains hunger strongly for one day. - Aayambil: Rasa‑parityag with ek-bhukti (one plain, tasteless meal). It restrains taste and craving, the root that repeatedly excites kashayas (passions).

• Sustainability

- Upvaas: Intense, but generally done for a single day at a time. - Aayambil: Can be practiced for many consecutive days (e.g., Navpad/Ayambil Oli), so it trains steady sanyam.

• Effect on senses

- Upvaas: Stops eating; hunger may be there, but taste-attachment can remain unchanged when normal eating resumes. - Aayambil: Daily, direct training of the tongue to accept nirasa (non‑tasty) food; this steadily weakens raga toward taste.

• For sadhana through the day

- Upvaas: Strength may dip for some, making steady svadhyaya/pratikraman harder. - Aayambil: Light, simple food supports samayik, svadhyaya, and seva through the day without stimulating the senses.

• Guidance in maryada

- Shwetambar and Digambar traditions both keep Aayambil as ek-bhukti of very plain, boiled food without taste-enhancers (no oil/ghee, milk/curd, sugar/jaggery, most spices; details per maryada). Upvaas is full fast for the day, often with only boiled water in fixed timings. The spiritual intent is identical—samyam and kashaya‑shaman.

Scriptural frame

• Both are among the classic tapas described in Jain texts: Upvaas aligns with anashan; Aayambil aligns primarily with rasa‑parityag and niyam of ek‑bhukti. Since attachment to taste is a powerful doorway to passions, consistent rasa‑tyag through Aayambil is highly praised. Yet, without right bhava, neither yields deep fruit.

Practical takeaway

• Don’t compare as a “competition.” If health or duty makes Upvaas difficult, Aayambil done with strict niyam and shuddha bhava brings strong punya and real inner change. If one can do both correctly under guru‑maryada, both are excellent.

For simple, authentic details of Ayambil and Ayambil Oli:

• Ayambil basics
• Ayambil niyam/menu style (plain, nirasa)
• Navpad/Ayambil Oli significance

Essence: When performed with right faith, right knowledge, and right conduct, both Upvaas and Aayambil purify karma. Aayambil is often considered especially effective for conquering attachment to taste, while Upvaas is especially strong for conquering hunger—both aim at kashaya‑kshaya and inner peace.