used 16-35 or new 16-35 II?

If 2.8 matters a lot, I think you have to go mark 2.

Sorry for the bad news The corner/edge sharpness and vignetting difference was pretty significant for me at least between the two. Prior to buying the mark 2 I borrowed a friends mark 1 for a few weeks, and that sealed the deal to not go with the mark 1.

Honestly I think UWA zoom is Canon's week point (compared to their other stuff) so I wouldn't want anything but the best they have since only the best and newest they have just barely gets them comparable to Nikon/Olympus/Sony UWA quality.

I love Canon, so don't want to get flamed, I just know that UWA is not as strong as say telephoto (where my Canon 70-200 ii might be the best lens I have ever used!), so I would go with the 16-35 mark 2 or a prime.