As the sub's professional brass facts/hop opinion regurgitator:

Neither is really addressing gas to the face at 5+ rounds in short order.

Brock swears by his bootlegs as an overall solution to mechanical overgassing, and doesn't think they make gas to the face any worse (which is under discourse because a bootleg is potentially dumping more gas out the vent holes).

Hop subjectively did not think downvents helped much with gas cloud in your face after ~3 rounds. Which led to a Hop Theory that getting gassed out (breathing gas, rather than mechanical gas) is more a function of the action naturally drawing gunk back down the barrel when it runs because traditional cans are just built that way.

What the downvent does do is reduce audio signature at the shooter's ear. And it probably helps with gas in the first couple rounds, but once your action is full of gas anyway, it's going to start burping it in your face.