I always choose the ISO. Unlike Neil, I use manual settings for sport like rugby/football. I used to use AV but once I gain experience and took note of the pros on here, I set a minimum SS, a suitable aperture, and an ISO to suit. As the light fades, its a balance between upping ISO and lowering SS or opening aperture.
When I've experimented with Auto ISO I find it unreliable. In bright sunlight when you'd think it would be as low as it goes (100) its often 400, and in dim light it doesn't go higher than 800.