What is an allternatve approach to many fields in an entity?
Like Clive said, testing would be the definitive answer. But if you look at the mysql queries that will have to be executed behind the scenes, I'm pretty sure you'd find that your bundle A with 30 fields will be simpler and faster than the bundle B option. You can use the Views Show Query module to output your view queries in both scenarios and compare them. In bundle B option, I'm pretty sure the mysql query will have to not only look up all of A's and B's fields but also look up the relationship between A and B bundles, an expensive additional JOIN.