There is not one answer to what is best. I say go with what you like and you can vary it for whatever you will be doing behind the boat. Same goes with speed. If you want to try a new trick, you may want to do it one wake, so you can go slower and/or with a longer rope. Or you can also go really short so that you don't have to go so fast to clear the wake. It is all about what makes you feel comfortable and safe.
I usually ride 60-65' and 20-21 depending on weight on the boat. But I can go 18 with a 50' rope to try something new.