My limited experience involved exactly this situation. The biggest problem with A (when he left) was that we ended up with a larger then necessary, very smart, un-maintenable app instead of the simple hack we actually needed. Because if was obviously smart we kept dragging it, when we should have scrapped it the day he left and rewrite from scratch a simpler one.
edit: B on the other hand progressed steadily for a couple of years and wrote a whole lot of code still in production. Needed debugging and refactoring but lives.
edit: B on the other hand progressed steadily for a couple of years and wrote a whole lot of code still in production. Needed debugging and refactoring but lives.