The OP's options are:
5dc + 70-200mm 2.8 IS II
5d3 + 70-200mm 2.8 non-IS
He currently has the 5dc and the non-IS lens. What does the "glass first" approach buy him? IS and a bit more sharpness? For sports, the IS won't help that much (since he'll need a higher shutter speed to freeze the action and is likely shooting from a stable position.) For weddings, the IS is helpful, but not nearly as much compared to the extra 2 to 4 stops in usable ISO. (After all, IS doesn't freeze subjects that are in motion.)
The "body first" approach gets him dramatically better AF (good for sports) and dramatically better low light IQ (great for weddings.) Also, as said before, the additional resolution mitigates the crop value of his 50D for sports. The 5d3 will also be superior for video than the T1i (plus providing him a nicer DOF due to the FF).
"Glass first" is just a rule of thumb, and it certainly doesn't mean "body never". In this case, the OP already has a very good L telephoto. His body, however, is outdated and his causing him to miss far more shots than the lack of IS on his zoom.
This is a no brainer--get the 5d3.