No, 64bit won't make a big difference - only more memory available to processes.
There is no clear preference.

I guess it depends more on the cameras/drivers used than the operating system.
Any driver that does BrightLight processing will use more CPU than plain and simple drivers just delivering the image captured from the camera.

If there's a processor with multiple cores, AbelCam will take advantage of them.
On my demo system with 14 "cameras" active, AbelCam uses only < 10% CPU.
It's a Intel i7 920 running at 2.67GHz.