I'd bet money that a photo taken with the 50mm f/1.8 and a 5d mkiii would be appreciably better than a photo taken with a 50mm f/1.2L and a t2i.
Having said that... that is one of the rare instances that this is true... very rare.
The reason I even bought that 50mm 1.8 to begin with was because of a Slickdeal
and I purchased it about a month before I settled on getting the 5d3. At the time I only had the body with a lens on order, but the 50mm 1.8 was already here.
The pictures shot with it are really good even 100% crops are awesome - probably due to the mk3 body. I wouldn't hesitate to use this lens (don't have it anymore) in daylight.