Rob;
Not an engineer, "But" it is generally understood i believe that a smaller tube with same wall thickness is stronger than a larger one, so removing metal from the outside should not weaken it as much as removing from the inside. In the case of two tubes of nominal 12ga size a difference of only .008" would of course be mostly inconsequential.
As to the 2nd question, I think the important point there would be which would be most likely to "Grow" & get worse. This I highly suspect would be the internal one. as it would be exposed to firing residue & resist proper cleaning more than the external one.
