CA1500 vs. SA1309 - Which one wins?

We turn off SA1309. The reasoning behind it is fairly weak.

Our team feels that the well-accepted practice of private members starting with underscores far outweighs the idea that someone might use a different editor on the code, which never happens in our shop anyway. As to providing an "immediate differentiation", the underscore does that as well.

If you really have developers that still use "m_" though and you still need to check for that, you could write a quick rule for just that.

Here is my usual solution:

class SomeClass
{
    int SomeField{get;set;}

    public SomeClass(int someField)
    {
        SomeField = someField;
    }
}