Size
The Saint Bernard is much larger: about 66 kg (145 lb) as an adult, against 41 kg (90 lb) for the Bernese Mountain Dog.
Shedding
Their coats shed differently: the Bernese Mountain Dog sheds through the year, the Saint Bernard moults most as the seasons change.
Temperament
They differ most on independence (Bernese Mountain Dog: keen to work with you; Saint Bernard: cooperative, with its own ideas) and on intelligence (Bernese Mountain Dog: learns readily, likes games; Saint Bernard: easy-going, simple routines).
Alike
Both are rated the same on affection: a lap or leaning dog.