Quick Answer
Choose Bali for: rice terraces, Hindu temples, yoga/wellness, surf culture, and a single-island-depth experience. Choose Thailand for: island hopping, street food variety, nightlife, Thai massage, and a multi-destination adventure. Both are affordable, but Thailand is slightly cheaper.
Beaches
Thailand wins for beach variety — from Railay's limestone cliffs to Koh Lipe's Maldives-like water. Thailand has hundreds of islands; Bali has a coastline but most beaches are good-not-great (exception: Nusa Penida and the Gili Islands nearby). Bali wins for surf — Uluwatu, Canggu, and Padang Padang are world-class. If your trip is mainly about beaches, Thailand offers more range.
Food
Thailand wins on street food — Bangkok and Chiang Mai's street stall culture is unmatched. Pad thai, green curry, mango sticky rice, and som tam at every corner for $1-2. Bali's food is more cafe-culture — excellent smoothie bowls and Western-influenced restaurants in Canggu and Seminyak, plus authentic warungs (local eateries) serving nasi goreng and babi guling (suckling pig). For pure food variety and value, Thailand.
Culture
Bali wins for cultural immersion. Hindu temples, daily offerings (canang sari), ceremonial dances, and rice terrace landscapes create a spiritual atmosphere you walk through every day. Thailand has incredible temples (especially Bangkok and Chiang Mai) but the culture is less omnipresent in tourist areas. Bali feels like a culture; Thailand feels like a country with culture.
Cost Comparison
Daily budget comparison (mid-range): Bali $50-80/day, Thailand $40-65/day. Thailand is 10-20% cheaper overall. Accommodation: similar. Food: Thailand cheaper (more street food options). Transport: Thailand cheaper (excellent trains, buses, domestic flights). Activities: similar. Alcohol: Thailand cheaper.
Verdict
First time in Southeast Asia, 2 weeks, want variety? → Thailand. Want depth over breadth, wellness focus, or learning to surf? → Bali. On a honeymoon or couple's trip? → Bali's villa culture edges it. Backpacking on a tight budget with friends? → Thailand's infrastructure is better. Many people do both — Bali for 1-2 weeks, then Thailand for 2-3 weeks. They complement rather than compete.