Hi,
This is just my opinion..
If you're really serious about saltwater, Just buy a big real tank. Not like those bio cubes, but a real tank that has a sump, good skimmer, fuge etc. Yes, you will spend a lot more, but look at the choices you have for fish. You may not see it now, but I bet you when you have corals, you will want more
If you buy a bio cube and you have MTS. I bet you you will spend money on a bigger tanks anyways in the future. So you will have to spend more money. So if you have a bigger tank,
# 1 you have more options.
# 2 You could make it more natural looking. (If you think bio cubes can make things look natural, it can't) (I was born in a country filled with reefs)
# 3 You'll have more choices for equipment
# 4 If you upgrade, you could still use some of the equipment.
# 5 It will be easier
I recommend you join a local reef aquarium society forums so you could trade and buy equipment/ livestock for cheap price.