There's been plenty of debates and arguments and such about the IQ between the two...so I will post my thoughts on all the other stuff. I just bought a new 50L, and I just got my Sigma back yesterday from a checkup and recalibration, so the Sigma should be performing as intended.
Build: One has a matte finish, the other is painted. Both are sturdy. The Canon is shorter, fatter, and heavier.
AF speed: The 50L is faster. Whether it's significantly faster is up for debate. The primary difference between the two is that the Sigma always hesitates and checks itself before it declares the object in-focus. The Canon does that much less, and when it does, it does it faster.
Focusing ring: The Sigma's feels scratchier (i.e. not as smooth) as the Canon. The Canon has more resistance, though that might go away with time - I remember the Sigma being more resistant when it was new. The Sigma focusing throw is ~90 degrees, the Canon's is ~150.
Other thoughts:
- From my not-so-rigorous testing involving measuring tape, the 50L transitions slightly faster from in-focus to out-of-focus. In other words, slightly less DoF at equivalent apertures.
- There's a massive "gap" between the front element and the filter threads on the 50L, which makes me wonder why Canon didn't put in a larger front element.
- I noticed a *hint* of backfocus when I was comparing DoF at f/1.4 on the 50L (I shot it at f/1.2 and f/1.4), but I didn't notice any when I took "real" shots at around 5-6ft at f/2 and f/1.4.
- The 50L's rear element is larger, if that's significant in any way.
- The 50L is 1/3 stop faster than f/1.4, not 1/2.
- The red dot on the 50L (to help mount the lens) is hidden behind a ridge, which is kind of annoying.
