The reason I personally would try to fit many eggs in the 20 instead of buying another is that many eggs may be bad or infertile so you can remove them by hatch date and end up with 24 or less but at least you tired more. When you get shipped eggs your odds also go down. Just my 2 cents.
I just ordered the Octagon 20 advance and all the talk about it being a nightmare to clean is a little unsettling. Are there instructions on what is best to use, etc? Thank you.