Robert, Jr. still has a chance to be a good player and the Mets can afford $20 million to take that chance. If he's a bust they can afford the hit.
Acuna is 4 years younger than Robert, but all he has is speed and some defensive versatility and not much else. He has no power. He's a bench player at best.
I guess the equivalent would have been the Tigers trading Hao-Yu Lee or Max Anderson for Robert, Jr. (although I think Anderson is a better hitting prospect than Acuna).
I wouldn't have had a problem with that trade, but Robert's $20 million salary probably kept the Tigers away. Also, perhaps the White Sox didn't want to trade within the division.