Your question assumes that opera possesses intrinsic merits, a contention with which several FReepers would take issue. ;)
Seriously, that's a good question and one which I must ponder. For the sake of consistency, if I had to do it right this second, I'm afraid I'd have to argue that the original work is superior, if for no other reason than its originality.
My stance is indeed quite harsh, but then, no one's ever accused me of being a big softie.
How about Shakespeare? Shakespeare borrowed a lot of his plots from other stories. Not many people claim his work suffers because of that, or that the originals were better.
Besides, Hollywood can't write anyway, so they might as well try to steal from those who can.
To me, that is what really shows intellectual bankruptcy.