If only there were one "ideal" spacing for children. I think its great to ask the question and ponder the responses, but in the end we all have to choose what is "best" for ourselves and our families.
As someone with experience with the 20-year spacing (my mother and uncle - the only children), I can say that I was not close to my cousins at all, being that they were adults when I was a child.
Other families had cousins who were close to each other. I didn't (there was significant spacing on the other side, too, only not as severe, but the same problem).
Therefore, I chose to have 3 year spacing to (potentially) solve the problem that I encountered growing up.
