A couple of things to keep in mind first off:
-Essentially any speargun is capable of killing any fish you care to hunt (Ex. a small single band gun pressed against a tuna's head will kill the thing), a lot of it boils down to personal preferences. A lot of this will come to effective range (bigger gun, longer range), components, and aesthetics (I like wood guns myself).
-As others have alluded to: different tools for different jobs. Personally, I would not use a gun I had chosen for grouper, cobia and snapper on flounder and other bottom fish. I personally own 2 guns and want more.
The best spearfisherman I know tends to only use guns he can buy used, with a single band, and he SLAYS fish. When I guided upland bird hunts, I would outshoot men with $5000+ dollar shotguns with my bought used $200 shotgun. My point is, a lot of it isn't the tool... it's the user.