If youre using the HotPlate as a load box only w/o a cab, it doesnt matter which one: 4, 8 or 16. Just set the amps ohmage accordingly.
If youre also going to use it as a vol/power attenuator, get one thats rated for your cab.
i.e. I have an 8 ohm cab, so I have the purple 8ohm HP.
The Webers ohmage is variable/selectable, but make sure you get one with enough wattage to support your amp.
The Megadump is rated for 200W, but they have others that are 50w, 100w, etc - with different options on them and priced accordingly.
Hot Plates are rated for 150 watt continuous, 185w max and have noise reduction circuitry.