If you want a VIA chipset for your S939 board, go with Abit.
The Abit AV8 series boards appear to be as stable as the Asus equivalents, and they have the added benifit of being perhaps the best overclocking S939 VIA based boards.
If you want an nVidia chipset for S939, well, umm, you'll probably have to pick another manufacturer.
Abit, last I checked wasn't going to use the current nVidia A64 chipsets.
Asus would win by default, but they don't yet have an NF3 based S939 board either.
Unless you want to pick another manufacturer, or wait for Asus, stick with the Abit AV8 solutions.