I voted dwarf cuttle (Pajama squid, perhaps? Maybe an offset daylight schedule, with a deep blue moonlight?).. because I've always wanted to try them, and never have... but the answer really has more to do with your experience and skill level. Cuttles can be a challenge. Mantis shrimp are easy, as long as you're keeping them in a species specific tank. Both are known to demonstrate some very interesting behaviors.
Thanks for the input. Offset daylight might be cool with them, I'm down for the challenge of cuttlefish. They'll have plenty of volume outside of the display to keep water quality good.
I’ve got a mantis tank. When I checked last time dwarf cuttlefish were scary expensive.
Cuttles would be cooler though if you could do it.
Saltwaterfish.com has them for $22 for CB babies, and eggs go for less on other sites I think - maybe you're thinking of flamboyant cuttlefish? They usually go for several hundred a piece.
3x2x2, about 90 gallons on a nearly 900-gallon system - lots of volume.
Get the cuttlefish first then the mantis. I love cuttlefish but they don't last long. Unless you get a breeding group they will be gone in a year.
I hatched 2 rounds of sepia bandensis but never got any to lay eggs themselves. If you have any questions feel free to ask.
There's an idea - I'd definitely try to get a breeding group going, though. I'll have about 140 gallons worth of quarantine space to work with, where I could keep babies.
Mantis, definitely. Cuttlefish are my fav sea creature of all time but I’d never keep one. Personally I would feel bad keeping one. Cephalopods in general are incredibly intelligent and I don’t think any captive envirnment can be adequate for them. I watched the flamboyant cuttles at an aquarium for about an hour. A few of them swam up and down along the tank walls the entire time. Couldn't help but think they wanted out or were simply bored. Octopuses are incredible escape artists, it’s one of the biggest reasons they are rare in the hobby.
Just something to consider...
There is definitely a downside to them. They would have lots of peaceful soft corals, macroalgaes, rock overhangs, and mangroves to swim around in, in a 3x2x2 tank - so maybe that would help with it?
Octopus was a consideration, but the reclusiveness of them (except the Coldwater species which would not fare well on my tropical system) combined with their excape artist nature just makes them unappealing to me. The cuttlefish, from my understanding, would be out in the open a bit more.
Drew