I agree with Tailgunner's conclusions... If the U3100 does not support 15kHz horizontal frequency, as the specs suggest, then it may not even display a picture (or just garbage) with the ArcadeVGA.
I am more impressed every day with the ArcadeVGA/W-G 7xxx combo I'm running in my cocktail. So much that I'm right on the verge of ditching the D9100 in my upright and getting another CGA arcade monitor and ArcadeVGA for it. Running the Windows desktop in an interlaced mode is completely a non-issue to me since I primarily just run MAME anyway, but even the few Windows-based games I've run on it look just fine to me. I would much rather that Galaga, Pac-Man, Time Pilot, etc. look as perfect as they can be since the reason I built my cabinet was to play arcade games. Running a couple other PC games is a bonus, and I would rather sacrifice the appearance of those games (if there is a sacrifice) than the real arcade games.
In short, I feel that the extra $100 or so the U3100 costs over a K7200 would be better spent elsewhere, assuming that the U3100 would even work properly with the ArcadeVGA.