Holly wrote:

I already have more than half a can of water-based poly from another
project, as well as oil-based left over from our recently installed hardwood
floors, so I'm set no matter what. I'm going with straight poly on some
additional cubes for other rooms, so I'll be doing them anyways. My goal is
to avoid buying anything for this particular project. We have so many
partial cans of paint, poly, primer, and everything else!

Since you've used both the water based and oil based poly, I imagine you
already know that the water based will not yellow the way the oil based will.
So if you have any concerns about the color's changing on the cubes over time,
you are better off with the water based. Personally, I always use the water
based for a variety of reasons, including yellowing by oil based, drying time,
ease of cleanup, smell, etc.