I personally think that if you're going to be housing goldfish, the best way to go is to put together your own tank and accessories.
I currently have two Eclipse tanks (small versions) with biowheel and filters in the hoods. They're great, but not made for goldfish and their massive amounts of waste. If you were going with tropicals or bettas, an Eclipse would be excellent! In the case of goldfish though, you wouldn't really be able to get the amount of filtration you need out of it and it's difficult to add filters to the back since everything is built in under the hood.
As for the nano cube, I don't have one of those, but I did recently purchase an Aquapod which is similar. Again, these setups are intended for small reef tanks, and aren't really made for goldfish either. I believe the nanocube like the aqua pod have a built in wet/dry filter (sump style kind of thing going on) in the back of the tank. What happens is you put in an underwater water pump, and water is first drawn through a sponge chamber... then flows through a biological media chamber or two, then pumped back into the viewing part of the aquarium. Again, this setup is great for fish who don't produce large amounts of waste and require a high filtration turnover. Tropicals and marine fish would be wonderful for this set up.
If you really are planning on getting one of the two though, I personally like the aquapod setup better. I am however a softie for sump and wet/dry filters.

It would be really difficult to put an HOB filter onto a nanotank or aquapod though, so that is something to keep in mind.
Hope this helped!
::edit::
sorry i forgot to address the gph of the nanocube comment. with the aquapod, the water turnover rate depends on the size of the underwater pump you put in that will fit in the allotted space. so it can vary, which is probably why they didn't mention it in the description for the tank.
as for glass or acrylic... they are both good. acrylic is lighter but easy to scratch; it offers good viewing angles since there are no siliconed edges. between the two, i think i prefer glass. it's a matter of preference i think. both are good... both can hold fish... haha i'm easy to please. so your choice!