TDP has test results. Not perfect results mind you, but results nonetheless.
The f/1.8 was only tested on the 1Ds2, while the f/2.0 lens on nearly everything *except* the 1Ds2, so not a direct comparision. That said, I would think that that 1DsIII's boosted resolution (21.1MP over 16.6MP) would show more problems, everything else being equal.
The actual results are not so kind. The f/1.8 is significantly softer, even when comparing f/2 vs. f/2. Unless you need the f/1.8 aperture, you're better off with the f/2. As they're so close to the same aperture, it's not going to make much difference in terms of light gathering, so I'd pretty much straight out suggest the 2.0, unless you need the added OOF blur... and it does indeed add blur.
Here's a blur comparision graph for head/shoulders portrait. Distance behind subject vs. % of frame the blur disc will cover.
As always, Search for "flickr pool (lens name)" to find out what people are able to do. If the 1.8 consistently blows away the 2.0 in your eyes, there you go.
Oh, and the f/2 has IS, which is always nice.
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