I think your concern should be focused on how hot it will get in the daytime. The small GH is really had to cool.
I have a 10X12 and had a lot of water in it but it was taking up to much space. I needed the space for my plants so I removed most of the water and the difference to the heating bill was not noticeable so next year I will use none or very little water.
If you put 100 gallons of water in the small GH then you will have the thing full of jugs.
Like my wife said we built the GH to grow plants not store water in it.
I know the thinking by most on this forum is fill it with water but if their is a savings it is so small that it is not worth it I would rather use the space for growing plants.
Buy the bigger one and sit it where you will have room to put another one end to end with the first one because even the 10X12 is too small

has anyone tried butting a 6x8 up against a window? Open window in daytime allows heat into house. Open window at night allows the same heat to come back into the GH...

?