This is a comparison of two Canon lenses at an identical focal length. It is not a perfect comparison by any means, as the lenses Zoom focal ranges differ. However both lenses were shot at 32mm being the long extreme on one and the short extreme on the other.
The shots are in the direction of the Kremlin, Moscow, Russia, from my apartment. You can just about see the Hotel Rossiya lettering of the hotel next to the Kremlin.
17-35/2.8L EF USM + EW-83CII Hood (Date of Manufacture 1999, USD $1000 used)
This lens, succeeded the 20-35/2.8L and was succeeded itself by the 16-35/2.8L. In the judgement of "L" lenses, this did not receive stellar reviews. The 16-35/2.8L is reputably somewhat better. However it is an "L" lens, and its certainly offers "L" performance.
28-70/3.5-4.5 II EF AFD + EW-68A hood (Date of Manufacture 1989, USD $100 used)
An unusual lens, in that it has the reputation of being one of the best if not the best none "L" zoom ever made. Although some attempt to compare to the "L" versions (28-80/2.8-4L and 28-70/2.8L) this is very tenous at best. However, one has to recall this lens is available for around 1/10th of the price of the latter two. Discontinued early 90's and replaced with inferior consumer lenses. A "classic" lens amongst Canon affecidenios. Getting rare.
Both lenses were shot at their fastest aperture (2.8 and 3.5 respectively) and f/8.0 at approx 32mm using a Slik Pro 400DX tripod with a Kodak DCS 560 (EOS 1n Hybrid) 6.1 MegaPixel DSLR @ ISO80 (the cameras lowest setting).
No Post processing (such as sharpening) was done on the files except for TIF to JPG conversion in PhotoDesk 184.108.40.206
My conclusion: Very similar, although the 28-70 seems sharper stopped down! A surprise.