Pai or Chiang Rai: which northern side trip makes more sense?
Pai is a slower scenic detour with a traveller scene; Chiang Rai is better when temples, landscape and a tighter onward route matter more. They are not interchangeable, and forcing both into a short stay usually weakens the north.
Answer
Pai makes sense when the point is to slow down, ride or be driven through mountain roads, hang around, eat casually and accept a more backpacker-shaped northern mood. Chiang Rai makes more sense when you want distinctive temples, a cleaner add-on from Chiang Mai, or a route that feels more like sightseeing than lingering. Neither is automatically the better side trip. The real question is whether you want atmosphere and time, or a more defined day-or-two extension. If the Chiang Mai stay is already short, adding either can make the north feel scattered. A good northern itinerary often improves by choosing one detour and giving the main base room to breathe.