That depends entirely on what type of coral you're talking about. The 24,28,29 gallon all in one cubes don't have very good stock PC lighting and you'll only be able to keep soft coral like zoanthids and mushrooms, and maybe some low light LPS, though not many varieties are low light. The problem with the larger nano tanks is that the lighting is weaker than the smaller setups. You only really have around 2.5 w/gallon (though that's not a good way to judge lighting, it's just what people understand, and when it comes to PC lighting it's the best way to gauge power). This is the BARE minimum that you can have to do the low light coral and you won't likely get much growth.
So, think about the type of coral you want and then go from there. If you're ok having soft coral and not much growth, then you'll be fine with the generic stock setup. If you plan on keeping LPS or SPS, consider getting one of the nanos with a MH fixture instead. There are quite a few people on the boards here that have one of those setups and they have some great stuff in their tanks.
