I think 15mm should be the standard. It became the standard for 120mm fans and there is no reason not to follow that standard.

The MCR420 uses this same 15mm standard per their drawing.

I can't see why you would want gaps between fans, 20mm just doesn't make sense to me unless there was a special fan that required the extra space.

15mm makes the most sense, cases should be designed to fit components not the other way around. You could potentially have an oblong hole in a case that would be compatible with multiple spacing standards, but you can't really do that with the radiator when you need threading.