The prime benefit with a more flexible fin is you get greater efficiency vs a bare foot for a minimal cost in increased resistance when kicking. I've found that to be generally true with split-fins as well because the split makes the "scoop" of the fin more flexible. I've never swam with the really long free-diving and/or monofins but their length/size means they can't be very stiff.
Stiff fins really shine when you care more about power/speed than reduced resistance. The fins that I typically use are stiffer fins, and I like them because I can kick softly to go slow, and I can kick hard/quickly to go fast. With more flexible fins I almost always reach a point where kicking harder doesn't actually make me go faster.
Really short fins are nice when you just want a boost when kicking, but don't want to deal with the pain of walking with duck feet. A good use case for this is boogie/body boarding because you spend a decent amount of time in shallower water where it is more comfortable to walk.
In practice (my experience so YMMV) this means that my split-fins/flexible fins let me go slow to medium speed with less energy, and my stiff fins let me go slow to fast speeds albeit with a bit more effort on the low end. Personally I like having the ability to push hard/fast if I need to so I always end up using my stiff fins and keep my split/long fins for loaners/backups. The design that I prefer is similar to the one below:
IMO you should probably go with a split-fin since your main concern is saving energy. They offer the least amount of resistance and let you go slow to medium speeds pretty easily. They are also more pleasant to kick IMO than a more traditional long rectangle flexible fin. The split-fins I have look similar to these: