Cavalier King Charles Spaniel vs Chihuahua

Heart disease isn't a risk for Cavaliers so much as a near-certainty — about half develop a heart murmur from mitral valve disease by age 5, and veterinary sources note that essentially every Cavalier that lives long enough will eventually develop it, making it the breed's leading cause of death.

Chihuahuas are genuinely fragile, with thin, easily broken leg bones and, in some dogs, a soft spot in the skull that can persist into adulthood, and despite the tiny, portable image they are one of the most surrendered breeds in American shelters, not a low-commitment accessory.