I picked up a copy of the f3.5 and have done a quick comparison with the f2.8. Tipod mounted, mirror lock up etc. using f5.6 on both the f2.8 and f3.5 M series lenses, on my ist DS, about 1.5m from the Economist newspaper, which is fairly shiny and reflective, as well as being well printed (and was in the bin for being a little dull).
I can't see any difference in contrast, and the f2.8 may be better! Neither managed to resolve the small text. Maybe the focal length of the f3.5 is slightly greater, as these are 100% crops, measuring exactly 999 pixels wide, without moving the tripod and the f3.5 pic looks slightly larger.
To quantify contrast, I took a 3 pixel average (using the curves dropper in GIMP) for the black area under the tie in David Cameron's suit, vs the white next to the text on the RHS. The f3.5 scored (171-17)/(171+17)=81.9% and the f2.8 scored (173-16)/(173+16)=83.1%. (If someone could write a GIMP plugin to calculate the average accutance for a picture, it would be very handy for contrast testing!).
Maybe there is an advantage on full frame to this f3.5 lens, and I have not shot with it out doors yet, so maybe it handles flare better? Suggestions are welcome for further tests as I don't want to get rid of the f2.8 too soon. One weakness I can see to this kind of testing, is that its dependent on my manual focussing skills without a split screen on the DSLR and my ist DS only goes to ISO 200.
Lens M 2.8/28, iso 200, 1/125 at f5.6 Lens M 3.5/28, iso 200, 1/125 at f5.6
Quick test shot, M 3.5/28 with 20mm hood, f5.6, 1/125 handheld