Modular, also referred to as a MPS(Modular Pulley System) allows you to change pulleys simply by unscrewing anywhere from 1 to 6 screws or nut to change pulleys. Most systems incorporate a "hub" that requires a specialty tool to remove the stock pulley to install the hub. Once that is complete you can change pulleys in a matter of minutes with a couple of typical toolbox tools. Magnuson also made a snout that had a keyed shaft that allowed quick change pulley times.
A press on pulley is simply that... You need a specialty tool to remove and install each pulley.
