The Willem 4.1 is better for EPROMs (more VPP options) and serial devices (4.5 doesn't support serial). A transistor switch for VCC is standard; the optional relay addresses voltage drop in the transistor when programming NMOS EPROMs or CMOS devices that draw more than 40mA Icc. On that design, you can only use the transistor or relay, not both. I don't use the relay for FWH/LPC devices because VCC is switched and the relay makes a racket.
The 4.5 is intended for FLASH (only have VPP=12.5V option, but has more VCC options), but doesn't support serial devices, DIP24, 27SF* erase. It has a TSOP48 socket which eliminates an expensive adapter if you want to program devices in that package.