The Balboa VS500Z supports one two speed pump, an ozone, and 12v spa light and an optional circulation pump.
Whereas the VS501Z will support a two speed pump, a single speed pump or blower, an ozone, a 12v spa light and an optional circulation pump.
Basically the two are very similar except that the 501Z will support an additional one speed pump or blower.
The best suggestion if you are doing a replacement is to purchase one of the Value Pack Retrofit Kits available through Balboa. You basically get everything you need for the replacement, including the topside, cables, and overlay.
You can find everything you need to know about making the replacement on the following page:
BALBOA VALUE PACK VS RETROFIT KIT Hope that helps!