I would get whichever one is available. If both are available, i would get the 1660supers.
With a motherboard like this:
you could pack 8 of them, remove the fans on the gpu and use 3 120mm fans to push through the heatsink
If I have this motherboard right now I will still use risers because there won't be enough space in between the graphic cards once you insert all the 8 gpu onboard and that's even if all the 8 gpu fits the motherboardWith a motherboard like this:
you could pack 8 of them, remove the fans on the gpu and use 3 120mm fans to push through the heatsink