I agree, go with optipac. It has more features than the happs and is a little cheeper. The only thing the happs has is three buttons per trackball, but the optipac makes up by letting you have two trackballs (with two buttons per), and/or 4 spinners (with the equiv of 1 button each).
I have both a USB optipac and a USB happs interface, and can't tell the difference between the them when playing TB games. Oh, I have a wico 3" TB, and yes it works on both.