Partitive when you are not talking about either mass/uncountable nouns OR a PART of something; accusative (looks like nominative or genitive) when you do talk about the whole thing or countable nouns.
Leipä can be both, countable and uncountable so you can say "Leipä on liian suolainen.", but then you are talking about the whole bread. "Suolaista" means you have taken just a bite or slice of it and based on that you can tell the bread is salty. Partitive is here definitely more common as leipä is usually seen as a mass noun.
Marja on the other hand is a countable noun, there just one berry and it's delicious. If you'd bite off just a tiny piece of the berry, then you could say "Tämä marja on herkullista."