It's all personal choice when you own and wear it yourself. What do you want? is the question. For me, the 21J movement is the smarter buy, but I feel better with a gen Swiss ETA. In terms of money, as already said, the market for a modded or fully franken watch is only on the rep forums.
Regardless what non-Rolex movement is used, as soon as the case is opened most people will know. But on the vintage Rolex GMT the DG3804B looks more like a Rolex movement than the ETA does, and would fool anyone at a glance that is not an expert. And for personal use, both will need service in about 5 years, the ETA service anywhere to $250, the DG you just remove, smack it with a hammer, and buy a new one for under $30.
But being 'called out' depends on many things, particularly you and what watch you are wearing. My Son wears a noobmariner I bought for him. He gets called out all the time, a college kid with a Rolex. But when he says his Dad got it for him it is accepted. I am older, drive a Mercedes, and never get called out. I've even been in the Rolex AD in Waikiki with my 16710, the owner held it in his hand trying to get me to let him change the insert to all black, or trade it for a 116710. He never questioned it.
My vintage watches are even better, always getting the 'oohs and ahhs', and I make the dials myself. People assume a lot.
Keep in mind that aftermarket parts sellers like Clark's sell substantially more stuff to gen owners and gen watchmakers than to the rep world. Many gen owners don't want to pay Rolex prices for a crystal when they can get a good quality that they'd never see the difference in much cheaper. Inserts, bracelets, clasps, hands, sell big for the aftermarket to gen owners.
We are the only picky ones, likely because we know what we can do.