Funny I was researching this lens yesterday: Here's the impression I got:
Old lens has less distortion, infact for 12mm it is very good. However it suffers from some softness in the corners. This is usually not an issue unless you really look, but it is there and needs to be mentioned.
New Lens: Sharper in the corners open wide is sharper than the 17-40: stopped down, 17-40 begins to catch up, but the sigma has huge width advantage should you need it, and infact is THE widest lens available on FF. Bad news is if you are into architectural photography, it has complex distortion patterns, not simple barrel fixed by PS but tri-modal, that distortion can be distracting; I am not sure if DxO has a fix for it yet, but I read somewhere that someone had a fix for it... I forget where.
Both old and new have their advantages, none is perfect, then again we ask for perfection but seldom really need it, unless we are specialists in a certain type of photography, in which case, you know which one to get.