Is one more practical than the other.
Ok. Don't have my fj yet but I do have a Subaru with a reverse cam and a cam in my hiace. Just because you have a screen to look at you should still look around. I have never used the screen alone. It's just a guide and makes it a bit easier to see what's back there.Sonar has some "Blind spots" to it (may miss picking up objects at certain angles), however, I would think that backing up by looking at the monitor is one-dimensional. When I back up with just the sonar, I can turn around, look at the different mirrors, sorat look "all around" with the sonar as another dimension.
FOr instance, if you are just looking at the monitor while backing up, you may miss a kid or car coming at you from the side.
Just my thoughts...
I also believe the FJ blind spots have actaully made me a more careful driver since I really have to be more diligent when backing up or changing lanes(!)