Definitely wait for the mark II!!! Its the best quality wide angle zoom for sure, the mark II has a super UD element, UD element, Ground Aspherical element, Gmo aspherical element.. And the Mark I has NO such elements which are super important. they really make a difference on top of what else they have learned in the 10 years or so the first one has been out. I think it deserves all that price. Finally there is a top quality wide zoom lens.
Compare the MTF charts between the two. thedigitalpicture.com has them quickly available for comparison on the 24-70 2.8L II USM review page.
I have only used primes for all focal lengths below 70mm, this is a highly anticipated lens for me!