It depends what you are working on, Med ball stuff can be used for strength or power. Usually people use them to develop power which means speed. I would recommend a 6-8 lb ball.
A 20 lb ball will probably be too heavy to work on explosive speed. A medball can be used to develop good sequencing in power/rotational movements.
Anyway, a 6 or 8 lb ball is what we usually use and then mix in a 10 lb ball sometimes.