Yeah the 21" depth is a little shallow to work with. That extra 3" can make all the difference in flow dynamics and scape options. It sounds like either the 120g or the 90 cube is going to be your best bet. The other question is how large of a sump do you want? With the 90g the sump options are going to be rather limited to about 20" and are all fairly cramped so return pump, heater, and skimmer options will be limited to what would fit in them. One thing I learned with having a smaller sump is how much of a pain in the butt they can be. I have to move the skimmer to swat out the filter sock, the heater is always in the way, and have to hunt to find short thin profile models that will fit. Then there's the return pump section. It is dreadfully tiny, so finding a DC pump large enough for the flow wanted/needed is also challenging and limits what can be used. Having a 48" stand, allows for a much larger sump with more room for equipment. Even though you are going minimalistic, you still want to start off with that you want instead of what will fit, as well as have the option available to add components later if you choose.
If you decide to go with the 90, another option is have a nice custom stand made that is wider and made to support it on one side or in the middle. This would allow for a larger sump vastly increasing equipment options, and a place to display other stuff if you choose.
