Size
The Tibetan Spaniel is much larger: about 5.5 kg (12 lb) as an adult, against 2.5 kg (5.5 lb) for the Japanese Chin.
Lifespan
In UK vet records the Tibetan Spaniel lives longer: a median of 15.2 years, against 12.5 for the Japanese Chin.
Shedding
Their coats shed differently: the Japanese Chin sheds through the year, the Tibetan Spaniel moults most as the seasons change.
Grooming
The Japanese Chin needs more grooming, daily brushing, where the Tibetan Spaniel needs brushing a few times a week.
Temperament
They differ most on affection (Japanese Chin: a lap or leaning dog; Tibetan Spaniel: affectionate on its own terms) and on independence (Japanese Chin: cooperative, with its own ideas; Tibetan Spaniel: decides for itself).
Alike
Both are rated the same on handler demand: suits a first-time owner.