The best way to stop action and make sure you get the highest shutter speed while getting *proper* exposure is aperture priority. I rarely shoot anything but aperture priority. In and out of shadows is too tough to manually move things at a fast speed(at least for me)

I think the only time I have shot shutter priority is when I needed a slow shutter speed panning.

Open up your lens wide open and you will always have the fastest shutter speed for that ISO. If your shutter speed is too low, bump your ISO until it is as fast as you like