In a previous setup I gained 1.5db subs up port back over subs and port back. In my current setup I gained 0.5db sub up ports to the driver's side over sub up ports back. This doesn't work in all circumstances though. Subs up and ports back is a good safe route. I agree that two 6" ports are perfect for that sub.
One thing that's also very important that hasn't been taken into consideration is the type of SUV. My team has 4 suv's on it (2 Explorers, a Blazer, and Jeep Cherokee) and not every one of them wanted the same firing position.