The cheaper paceman units that only throw the 50g balls are ok, but you will have to replace the wheel fairly regularly. It has a light rubber coating that wears off after a while, and makes the machine vibrate. Fairly easy to change, but not something you have to do on units that are designed to throw heavy balls.
The light balls don't have the momentum, and slow down much quicker through the air than heavy balls. Our club has a Juggs machine, and while using the 140g yellow balls, I occasionally throw in a 50g ball, which works out as a great slower ball to make the batters think.
The light balls will also swing more than heavy balls