IMHO, another battery increase weight, more wiring and cost. Putting inverter result in noise, the 'jet sound'. When you rev the car, it's like sending a jet flying. Cool but annoying after a while. Anyway, a DC to DC is more energy efficient, my two cents though.

If want to try first but don't want to invest too much, go for the inverter first. Just make sure everything wired correctly. And safety first, eye on the road.