I've gone back to using the 50mm more and plan on getting a used 5DmII and just leave the 50mm on it all the time
The Sigma 50/1.4 is said to be stellar on crop but to have iq issues on full frame - let us know your experiences after you switch :-p
I'm a recent "switcher" from a 50D to 5D2 and though I read all the stuff about the Sigma 50 not being any good on FF, I actually found it to be very good.
And those findings are corroborated when I go to DXO and compare the field maps for the Canon 1.4, Sigma 1.4 and Canon 1.2. You'll see the Sigma is similar to the Canon 1.2 in terms of how sharpness works from center to edge at different f stops and how vignetting works at the different f stops.
Mine is a fairly recent copy and exhibits a small amount of focus shift from 1.4 to 3.2, but small enough to not be relevant with proper AFMA (it needed +5). Mine is also pretty good about locking focus consistently as long as I don't put a filter on it. I had fits with the AF on the 50D when using a B+W MRC IR on it for some reason, have not tested yet with filter on the 5D2.
I have no desire to change it for something else whether on FF or crop and for me the next lens may be a 135 L.