Switching 12V/6A with an AVR - MOSFET or relay or both?
I would also use a relay assuming the heater element has to switched on/off only a couple of times a minute (or less frequently). Yes you can just use a transistor to switch the relay from the AVR, but I would use a MOSFET as that would make it even more easy, look here under "2 Interface with MOSFET output". That is what you need. There is already a flyback diode in the 2n7000 so no need to worry about that.