I think part of the difference is that in Card a lot of the plot tension comes from overly contrived absurdities such as:
- a sentient microscopic disease body,
- the proposition that one shipload of pre industrial aliens can wipe out the whole human race when presumably most of the inhabited planets they might try to destroy would shoot them down before they'd ever get near enough to land or deposit their viral payload,
- a virus that can wipe out the whole human race yet one isolated biologist in a backwoods community, limited facilities, and no networked system of fellow researchers can develop a bacterial antidote capable of eliminating the virus from a whole planet without undue difficulty
- A husband whose body will dissolve if his wife isn't more cuddly and nice
- An instinctive science search for ways to survive that involve killing as many people as possible instead of as few as possible
- local sewer systems which are controlled by an interstellar computer network, that can be run independently without difficulty but only if the connection to the instarstellar network is blown to bits
Heinlein has a lot of moral tension as well and some of it is probably contrived. However, I don't recall feeling any of it is being forced into black and white categories that don't fit them even remotely. Heinlein also feels like someone I can have a good argument with-wheras since Card is more preaching than arguing so I can't have an argument with him about where's he's right and wrong. He's not inviting argument. So far, I prefer Heinlein to Card.