Both are good. They are not interchangeable.
Chiang Mai and Hoi An have been the two default answers for "where do I slow down for a month in Southeast Asia" for fifteen years. The reasons are real: both are warm, affordable, walkable, calorically generous, and built around independent merchants and small kitchens rather than malls. But the textures are different, and the practical considerations — air quality, water, scene, pace — separate them more sharply than first-time visitors expect.
The burning-season problem
We have to mention this first because it is the single most important practical factor and the one nomad blogs routinely underplay. From roughly mid-February through late April, Chiang Mai's air quality collapses due to agricultural burning across northern Thailand, Laos, and Myanmar. PM2.5 readings stay above 150 for most of the period, often above 200. This is genuinely bad for health and makes outdoor yoga, running, and beach-adjacent activity unwise. Hoi An, on the central coast, gets sea breezes from the South China Sea and has no equivalent air-quality window. If you are planning February to April travel, this is the deciding factor.
The scene question
Chiang Mai's digital-nomad scene has been mature since the mid-2010s. Punspace, Camp, and a dozen smaller coworking spaces; coliving outfits; weekly meetups; a large floating population of remote workers. If your reason for slow travel is community, Chiang Mai is the better answer. Hoi An has a smaller version of all of this — a handful of coworking cafes, a few coliving setups in Cẩm Nam and Cẩm Thanh, occasional meetups — but the community is roughly a tenth of the scale. If your reason for slow travel is quiet, Hoi An is the better answer.
Food, water, and rhythm
Both cities reward eating like a local. Chiang Mai's signature is northern Thai — khao soi, sai oua, nam prik, sticky rice — with strong Western and plant-based scenes layered over the top. Hoi An's signature is Central Vietnamese — cao lầu, mì Quảng, white rose dumplings, herbal broths — with strong Vietnamese vegan (chay) options and a remarkably high-quality seafood scene at An Bàng. Hoi An's water — both the Thu Bồn River and the sea — does work that mountains can't replicate. Chiang Mai's mountains do work that rivers can't replicate. The choice is partly which element you want near you.
Wellness, honestly
Both have serious wellness depth. Chiang Mai is the source for traditional Thai massage in its Wat Pho lineage form and has the largest Lanna herbal tradition. Hoi An offers Kerala-trained Ayurvedic practitioners running classical protocols (warm-oil abhyanga, Shirodhara, herbal steam) and the Central Vietnamese herbal tradition. Neither is better in absolute terms — they are different lineages doing different work. For the long-stay traveller who wants treatments built into their week, both are honest answers.