I've got the Sigma 24 1.8, and to be honest I haven't used as much as I'd like. My use for it is mostly landscapes, particularly long exposures with a 9stopper, and for the awesome perspective that the pseudo macro focusing distances affordd with the wide angle; you can pretty much get it to focus on stuff inside the lens hood.
Tbh I don't use it enough to really give a comment but the 28 1.8 is meant to be a very nice lens; I just wanted something that had the close focus of the Sigma. I wouldn't really recommend the Sigma unless you wanted that close focus as there is a little bit of wobble in the barrel which doesn't feel great and the autofocus is very slow on the 5D, but as a perspective it's something you always want to have covered.
Consider if you really need a prime for it though; I did to get the close focus, but if you don't need it wide open (i.e. you're using it for landscapes mainly) consider something like a Canon 20-35 or Canon 17-40L instead for a little more range.