Oh, lava pit for sure.
Here is the deal with lava: you need access to a source (so build wisely) and ventilation (unless you are undead). The latter can be powered by the lava itself – if you build wisely, much or all of your operations can be powered by the lava.
Sharks are gorgeous. They are effective. But they also need lots and lots and lots of room, and a constant source of food. Sure you say “I’ll just throw invaders in” but they don’t come on schedule, and then you will be sitting up at night worrying, “Are the sharks hungry? Are they getting the correct balance of nutrients?”
This doesn’t even include shark treats and toys.
You go into Petsharksmart and say “I just need a bag of fish-kibble because the heroes aren’t eating their vitamins” (all angry mental tone) and you walk out with a $100,000 bill and several deliveries of gigantic chew toys and…
…just…
…go with the lava pit while budgets are tight.