I've used both, the 100-300 on a 5D and the 80-200 on a DMR. I found the Contax lens awkward
to use. Maybe just because all other Contax are smaller. The accutance and color were OK but the Leica zoom is just the best. I think it is the sleeper of the R lenses.
I use the 100-300 together with a 5D and like it very much as a travel and landscape lens. It's prime sharp at the wider end (at f5.6 at least as good as my Contax 100 2.0) and loses a bit in the 250-300 range (but is still good). The lens is optimized for wide open use and performs best at 5.6-8.0.
I like the compactness of the 100-300, especially for a 300mm lens. But it's not a light lens. Due to the 4.5-5.6 wide-open apertures I found the lens sometimes difficult to focus.