I would think nobody will ever know which is stronger.
My guess is to prove it a test would have to be done with maybe 10 welded frames and 10 lugged frames- all other variables held constant. And the frames would be put in a machine that simulates a very strong rider grinding it out. And run the machine until the frames break.
I bet that’s never going to happen. And I bet both ways are strong enough.
I think lugs are cool looking.