The 16-35 MkII is supposed to be much much better than the MkI.
See the following review:
http://www.16-9.net/lens_tests/canon1635ii_a/c1635ii_a11.html
If you're looking at the MkI for budget reasons, you may be better of with the 17-40 f/4.0, which is cheaper and also reputedly optically better than the 16-35 MkI.
Except it's not 2.8.
If you're looking at the MkI for budget reasons, you may be better of with the 17-40 f/4.0, which is cheaper and also reputedly optically better than the 16-35 MkI.