PWM Control of Hotwire Power Supply
The schematic you posted (dial a speed) is suitable for your needs. The stated FET(IRF510) tolerates 100V 4A, so it could handle your hot wire with proper driving/cooling. If needed you can find many alternative FETs with lower voltage rating and higher current rating really cheap.

A relay would theoretically work for driving a low frequency PWM, maybe less than 1Hz (I believe you dont need high frequency). However in practice it is never done and I would avoid that route, especially if you have already built FET PWM controllers.