Auto-selecting the radio button prioritizes efficiency over obviousness. Disabling the text field prioritizes obviousness over efficiency.
As suggested in bug 84908, bug 99519, and bug 202407, when the stuff dependent on a radio button amounts to little more than a single text field, I think it's ok to prioritize efficiency over obviousness.
Auto-selecting the radio button prioritizes efficiency over obviousness. Disabling the text field prioritizes obviousness over efficiency.
As suggested in bug 84908, bug 99519, and bug 202407, when the stuff dependent on a radio button amounts to little more than a single text field, I think it's ok to prioritize efficiency over obviousness.