Upload & Sell: Off
My experience has been that higher ISO settings on my 5D II (400 instead of 100, for instance) make little difference when you have plenty of light—as you would from your flash. But when you resort to higher ISO because you are pushing to get images in low natural light, shadow problems show up, and generally force at least some exposure compensation. Quite often a plus 1/3 stop compensation suppresses the problems.
As long as you are keeping your exposures within reason, however, even without compensation you might not see anything troubling unless you make large prints. On the 5D II large prints can be very demanding, even at ISO 200.
I think that's generally true for all digital cameras, but probably to varying degrees. I hope the 5D III does better, because I'm taking delivery of one tomorrow. But I actually don't expect it to be very different. Nothing I have seen in images published online convinces me that there is a major noise advantage for the 5D III, despite what I read in comments. Love to be proved wrong.