I can do a lot more on free weights than on the machine too. I think it is from the range of motion though for me. With a barbell I can only bring it down to the height of my chest and I start with the weights at the top, with the machine the start position is further back (down if its a lying machine) than I could go with a bar and since I don't work heavy in that range very often I cannot get as much going.
Free weights will work your forearms and other muscles a bit at the same time to keep the load balanced. Many people prefer free weights for one reason or another, almost as many prefer machines for other reasons, really the most important thing is to just do it, either way you will see way more results than doing nothing.