I'd use "innovative" because "groundbreaking" is something more natural with "discovery". An idea needs an implementation, or, at least, a more complete form (theory?) to "break the ground". Just my opinion...
I prefer groundbreaking. It emphasizes how revolutionary it was. An innovative idea might save some money, come up with some new perspective, but it doesn't have nearly the same degree of "Wow!" as does a groundbreaking one.