tuz

What he said.

Internal derailleur routing has been done and is still done for either looks or for (pseudo) aero reasons. It does add a few bends to the cable and may add some friction which could affect indexing. Never tried it. The routing scheme is a bit more complicated than for a rear brake, which is why it is rarely done I suppose.

For the rack, the external bosses are easier to put on. The internal should be equally strong (the hole is reinforced by the boss) but require more work. Functionally they are the same, internal is neater.

I read that lugged has a slight edge strength-wise vs. welding. But by the time either one breaks, the bike is probably FUBAR anyway so any strength difference is mostly theoretical.