I own both and they both have their own strengths and weaknesses. I have also compared it to the 14mm f/2.8 II. Here are my thoughts on each:
Canon EF 16-35 f/2.8 II USM
Pros:
- You can use filters
- Center is sharp at just about all focal lengths/apertures
- Corners sharp beyond f/8, but never quite as sharp as center
- Distortion pretty good considering how wide it is
- 24-35mm focal range is VERY useful to have
- Built like a tank
Cons:
- After using the 12-24, 16mm seems narrow and constraining
- 82mm filters ain't cheap, but if you can't afford the hub caps, you can't afford the car, right?
- Hood is huge and takes up a lot of room in your bag
Conclusion: A great all-around lens for environmental portraits, events, landscapes, and more.
Sigma 12-24mm F4.5-5.6 DG HSM II
Pros:
- 12mm is f!@#$ing WIDE!
- Pretty sharp at f/11-16
- Distortion is high, but can be corrected in DxO / PS / LR
- You can achieve very unique looks with it
- Photos from it have been very popular with my clients
- Built solid, but not like L series
- CA about average - a bit higher in the corners
Cons:
- 12-24 isn't much of a range
- No filters without using one of those crazy rigs (like the Nikon 14-24)
- Need to be very careful with front element
- CA pretty high in extreme corners
Conclusion: Great lens for landscapes and architecture, but you need to stop it down to f/11 pretty much all the time. No other lens can give you this wide of (rectilinear views) so you will get very unique photos. Difficult to master shooting at 12mm.
The 14mm f/2.8 II is a great lens, but very pricey. It's FOV is nowhere near as wide as 12mm (14mm covers roughly 2/3 of the 12mm frame). It's MUCH more compact than the other two lenses. The distortion is incredibly low and there is almost no CA.
The 16-35 II is the most useful and versatile lens. The 12-24 II is great for really unique shots but isn't as useful for subjects other than landscapes and architecture. Stopping down to f/11 isn't a big deal because you can hand hold at 1/15s!
The 14mm II offers the best image quality, but can't use filters and is "only" a 14mm lens and is generally too wide for all but the most unusual portraits.
I'd probably go with the 16-35 II, unless you want 12mm for really unique shots, or 14mm is the only focal length you need and you have to have the ultimate IQ.
P.S. Of course you could always to the Nikon 14-24 with a Novoflex adapter like Marc Adamus and many others...