On a fretless, does it play better with a flatter (14-20) neck radius, does it make no diference, does it make it harder? I'm building a fretless, and will most likely copy the specs of a fender J-bass neck. SHould I have a higher radius?

That is a personal preference question. You can use whatever radius suits you best. Upright basses has a severe radius, and I know there are people that like a straight board, no radius. Find what radius is most comfortable to you.