I've since lost the napkin, but here's what we figured out.
Comments are welcome .
The difference comes in the chance that the loss of a second drive from the system will result in the failure of the whole system. This would be both drives in any numbered pair in the diagram. The math gets more complicated when you have more than two elements to a mirror. Since that's a rare configuration, I haven't bothered to figure out the equations. If someone else would like to, I'll be glad to post them here.