Have you considered maybe something with shorter shanks so that even if you do have a Hulk moment, it's not quite so powerful in his mouth? Or maybe something with a lower port so that the pressure is spread evenly over his tongue and his bars instead of all of it being on one particular part of his mouth.
I really don't think either one of those is milder than the other, they just work differently. The second one will put more pressure on the outside of his bars whereas the first one will put it more evenly on the front of his bars.
Personally, though, I am not a big fan of correction bits (the second you posted). My older horses like to carry the bit themselves and with something like that, they can't carry it on their tongue like they prefer.
I don't know if there are any rules about what length the shanks have to be or what the mouth has to be like but I have just recently begun using this bit and I really like it. It's mild, it feels nice in my hands, and it still gives the individual side control without collapsing on their mouth. AT Low Port Loose Cheek Low Port Western Bit 5in - Horse.com
Anyway, for the basics of how a curb works...
The pressure in the horse's mouth is amplified according to how long the shanks and purchase are. If the purchase is 1 inch (from the mouth to the bridle ring) and the shank is 4 inches (from the mouth to the rein ring), then the pressure that the horse feels is 4 times more powerful than the pressure that you exert on the reins. Like the way a snaffle is a 1:1 ratio (for each one pound exerted, they feel one pound), this bit would be a 1:4 ratio (for each pound you pull, they feel 4).
For a long shanked bit, just picking up some of the slack out of the reins is about the same as taking light contact in a snaffle.
The pressure is spread out to different parts of the head as well. The pressure on the curb chain helps to encourage the horse to break at the poll and the poll pressure encourages the horse to lower his head.
Ideally, on a well trained horse in a long shanked bit, raising your hand a couple of inches should cue them to both drop their head and tuck their nose.
Hmm, I think that's about it.