If you want 3 or 4 fish then the larger systems are almost a must have. My friend has the 29 gallon bio cube and he has no complaints. He has sun daisies, frogspawn, chili coral, pulsing xenia, finger leather?, and another polyp I think a leather. Stock everything and he is very happy. He has a coral banded shrimp, pom pom crab, dragon goby, and two ocellarus clowns. He had a spotted mandarin but it blew a hole in it's side, it looked like it over ate because you could see the pods in its stomach... I personally think it's overloaded but no ill effects have been seen yet. (FYI i have a larger tank he can move stuff into)
If you want heavy coral or attempt things like anemones and clams then you would probably want the aquapod.
Then again the nanocubes are good too, just stock lighting limits to softies in general.
Just depends on what you want IN your tank.
I hope that helps.