Both are Jain Śvetāmbara Sthānakvāsī groups, so in doctrine and basic practice they are very similar. The main differences are organizational, history and acharya‑paramparā.

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1. What is Sādhumārgī Saṅgh?

• “Sādhumārgī” = those who follow the path of sādhus (ascetics).
• It is a Sthānakvāsī, non‑idol‑worshipping Jain saṅgh.
• Monks and nuns wear white clothes, use muhpaṭṭī, do pratikraman, svādhyāy, dhyān, etc.
• Worship is done in simple halls (sthanak/upāśray), without mūrtis.
• It has its own Acharya and senior sadhus, own internal rules and management, and its own network of sanghs in places like Rajasthan, Gujarat, etc.

You can read more here

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2. What is Śramaṇ Saṅgh (Vardhman Sthānakvāsī Jain Śrāvak Saṅgh)?

When people say “Śramaṇ Saṅgh” today, they usually mean the Vardhman Sthānakvāsī Jain Śrāvak Saṅgh:

• Also Śvetāmbara Sthānakvāsī, non‑idol‑worshipping.
• Same basic way of worship: sthanak/upāśray, scriptural study, pratikraman, etc.
• Formally organized in 1952.
• Acharya Atmaramji was the first Acharya of this Śramaṇ Saṅgh.
• Later famous Acharyas include Acharya Anand Rishiji, and at present Acharya Shivmuni ji is recognized as Acharya of this Saṅgh.
• It also has its own internal rules, management and network of upāśrayas.

A short, related reference is here

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3. Main similarities

• Both are Śvetāmbara Sthānakvāsī Jains.
• Both are an‑mūrtipūjak (non‑idolatrous).
• Same core Jain principles: ahimsā, satya, asteya, brahmacharya, aparigraha.
• Same main lay‑vows, same emphasis on pratikraman, samayik, svādhyāy, sādhu‑seva.

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4. Main differences (simple view)

1. Organizational identity

- Sādhumārgī Saṅgh = one independent Sthānakvāsī sangh with its own structure. - Śramaṇ Saṅgh = another Sthānakvāsī sangh (Vardhman Sthānakvāsī Jain Śrāvak Saṅgh) with its own structure.

1. Acharya‑paramparā (line of leaders)

- Each has its own Acharya and guru‑lineage. - Names of Acharyas, their decisions, and style of guidance will differ.

1. Practical rules and style

- Small differences can appear in: - Exact pratikraman order and tunes - Rules about mic, travel, upāśray discipline, etc. - These are maryādā‑based (discipline‑based) differences decided by each sangh’s Acharya, not differences in basic Jain siddhānt.

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5. Spiritually

From the view of Jain dharma and moksha‑marg, both are walking the same path – following Bhagwan Mahavira’s teachings through different organized sanghs. The choice between Sādhumārgī Saṅgh and Śramaṇ Saṅgh is usually family, locality, and guru‑bhakti based, not a difference in fundamental Jain beliefs.