One is saying “my team is good,” while the other is saying “your team is bad.” The latter is a penalty because it is deemed disrespectful by the rules committee.
If this is the case then every time players get in each other's faces talking boat during the game they should be flagged for taunting since their words or most likely far more incisive than "your team is bad," which is clearly so disrespectful it has no place on the gridiron. That clearly doesn't happen, though, so neither should it with hand signals unless used in an especially provocative way that also would have drawn a taunting foul without the hand signal.
I think both hand signals are saying "my team is better than your team" after a big play and if you don't like seeing the other team celebrate in either way you can try harder to do something about it.
I'm not trying to sound like some macho brochacho here who thinks there should be no rules for taunting or limiting celebrations. Honestly, the NFL celebrations irritate the hell out of me. They aren't celebrating anymore, they're just being idiots. Just be excited that you made a great play and get ready for the next one. But to me being excited about making a great play includes some form of celebration. I see nothing wrong with either of these hand gestures by either team unless done in a demonstrably taunting manner.
Take the Will Grier horns down that launched this controversy as an example. He's got his fans in front of him when he does it. He's saying to his fans, "we're better than these guys so get excited" when he flashes the hand sign. It's not a taunt. It wasn't untrue. It wasn't malicious. It wasn't disproportionate to a normal celebration that is well within the rules. Any type of celebration there can be construed as him saying "your team is bad" since the only reason he's celebrating is because his team out performed theirs. This whole debate is extremely weak, IMO.
Darn UT and Mensa.