Ideally, there is not going to be a huge difference withe the Pressure Pro and the Simpson, both are high quality models. The Pressure Pro units do give you a little more tlc being they are all built as the order comes in and don't come off of an assembly line, but the quality is still there on either unit.
The benefits of a belt driven unit is the pump is going to run at half the RPM as a direct drive. A belt driven pump is going to run at an RPM of about 1740 and a direct drive runs at or around 3250-3600. This is going to put less pressure on the pump and give you a longer lasting product. In alot of cases it really can depend on how much you use the unit. If you are just looking for a high quality pressure washer for your home and some other odd jobs, a direct drive unit is more than fine, but if you are using for business and multiple tasks and it will be getting alot of use, a belt drive is a better way to go.
Thanks,
Pressure Washers Direct