What is the difference between the standard model and the F version?

The F suffix denotes the anti-static build. In the standard model the cabinet is finished in ivory white; the F version uses a blue-grey finish and holds surface resistivity between 10⁶ and 10⁹ Ω, with a 1 MΩ earth lead fitted as standard. Everything else is shared: the same door lock, 5 mm tempered glass window, cold-rolled steel body, LCD display, sensor and dehumidifier. The F build is what you specify when handling ESD-sensitive components, where a static discharge would do the damage that the humidity control is preventing.