Another advantage of the ball and/or bench is that you are using your stabiliser muscles at the same time too (hope i got the name right there..)
This meaning that unlike on a regular weights machine where you are only using the exact movement, using free weights on a bench or exercise ball requires you to keep your balance and keep the weight lifting motion stable at the same time.

You should try out in a gym some time how much you can bench press using a weights machine and then try the same thing on a weights bench......its going to be a big drop in weight!