Switch current rating - actually current or really wattage rating?
The mechanical design of the switch affects both the voltage and current ratings. The higher the voltage, the greater the chance of arcing as the switch opens. The arcing can reduce the switch reliability over time by inducing oxidation of the surface of the switch contacts and hence increasing the resistance of the contacts. In addition, the voltage rating is provided to offer an insulation guarantee. The switch must be safe to operate at the rated voltage and pose no risk of shock to the user. 

Too high a current results in a voltage drop across the switch reducing its functionality (an ideal switch should have 0 resistance and hence 0 voltage drop across it). 

In your load circuit, it appears you'd like to use one switch per resistor. Since the resistors are in parallel, the maximum current a switch will see will be 35A/2 = 17.5A when you turn on a second parallel resistor. The 20A switch should suffice. when a third switch is turned on, the current in each switch reduces to 35A/3 = 11.67A. 

To summarize, for your application, you need only consider the current rating of the switch. If your application is mass-market, to minimize cost you would perhaps want a lower voltage-rated switch which will be less expensive.