Biocubes are nice, but with the stock lighting, you can't have every type of coral you might want, so you will probably want to upgrade them later as the addiction sets in.
I have a 28g Nano Cube it came with an upgraded metal halide light, which saved me some money in one upgrade, but I still ended up dropping a lot of money on upgrading pumps, which adds to heat, which made me get a chiller in summer ($$), and I upgraded my skimmer. In hindsight, I might have just held out a little longer for a nice used set-up that had all the upgrades already, I might have paid more up front, but less in total
It almost never ends in wanting something just a little better for your tank at least now I have a little better idea of I want if I ever upgrade tanks.