Lots of folks use the 70-200 2.8 for weddings on a FF camera, but switching from a 70-300 on a crop to a 70-200 on a 5d version might be too different for ya. One good thing about the 5d3 is you can crank the ISO up a bit more than the 7d if you do decide to keep the 70-300. Maybe use the 7d and the 17-55 as your other combo. Maybe buy a flash or faster prime lens instead are my thoughts.