In my experience Lower jaw species are less active then upper jaw species just sitting on the the bottom 95% of the time often burried if the substrate allows it), late at night they will migrate out and sit on top of driftwood but that's about as high up as they like to go. Upper jaw species are also bottom dwellers but spend more time cursing exploring/searching for food and exploring the mid/upper levels of the tank from time to time.
I find the lower jaws get along better with each other, less fin nipping they like to sit together in piles, While upper jaws tend to move around and hide separately more often, they also nip at each others fin's and even display territorial behavior. Upper jaw species also cruise around and check out or even nip/pester other tank mates from time to time as well. While lower jaws seem happy just laying side by side all day long showing no interest in other fish unless they see it as food.
Upper jaw species seem to be much more nippy toward new tank mates, almost testing to see if anything new is food but it's only turns into an attack if they find something suitable to eat. Low jaws on the other hand seem to ignore tank mates UNLESS it's something they want to eat in which case there is no nipping it's an explosive ambush attack not nips to test first.
Feeding behavior is almost opposite when it isn't live prey, With frozen food/pelets my upperjaw species are still quick to search out and eat food, some even surface to get at it faster. Lower jaws on the other had tend to be slow and methodical in searching for food then they'll sniff what they find for a while before eating it. So in a mix My experience is that the upper jaw species get the majority of the food.
Low jaw species also seem to make a lot more noise when breaching for air, I believe this is because they dont like to remain at the surface, while my upper jaw species calmly swim up for air even spend time chilling in the floating cover at times.
Most of the behavior difference seem to lead down to one main thing - Low jaw species are strictly ambush predators, While the upper jaw species are more active hunting predators.
As for prey size, i have seen upper or lower jaw both take down large prey, I tell everyone picture the bichir as if it were hollow, If you think the tank mate can fit indie that space the bichir will think the same thing! with that said if you consider the feeding behaviour i can see how that could lead to the conclusion that lower jaws take bigger prey because an upper jaw might decide after biting on that the prey isnt worth it and let go, where if the lower jaw launches an attack it's and latches on it wont change it's mind.
It's just the way they are wired in the wild an ambush pred can't afford to wait for the next fish to swim by, an active hunter has a good chance of finding something easier to swallow.
Hope that helps,
