I've got both. I've shot the 20mm since film days on a pair of EOS5's
Here's what you'll find.
Both have falloff that is considerable wide open, even on a 1.6 crop. The 20 is worse.
On a 5d2, the peripheral adjustment almost corrects that fall off 100 percent. Think in terms of 95+ percent, bare bare perception of falloff, either lens.
Stop down either lens one stop, and fall off is greatly reduced. If you shoot either stopped down more than a stop, you'll not even notice any falloff, except on bare flat surfaces shot straight on. Its a non-issue for the vast majority of shooting, if you close the lens more than a stop from wide open.
Both show a little flare, but its not bad. The only wide lens that I own that does not show ANY flare, is the 28/2.0 Summicron Aspheric... but its $2000 more than either of those lenses. Wides tend to have a little flare, neither of these are bad.
Sharpness - both are a little soft in the corners wide open, and sharpen up nicely with two stops stop down on FF or a stop down on 1.6 crop. Centers are very sharp. Contrast is excellent in the center, and like sharpness, a little stop down makes it excellent. Color rendition is the same. CA doesn't bother me in my shooting, but... both have a little residual CA, the 28/1.8 worse in that regard but not terrible.
Honestly, they're lenses that work like we'd expect them too. Relatively fast optics, showing typical wide open edge softness, that clears up well with a little stopping down.
My overall opinion - they're not punchy like Leica current Leica lenses, but... they're decently good lenses, either one. Choose by your focal needs. I liked both on a 1.6 crop - the 28 shot sort of like 50, and the 20 shot sort of like a 35 (my favorite focal length). Both are ok in low light, with the 28 a bit faster, naturally.