I just did this myself.
Mounting it inside has many advantages. You don't have to route the wire all the way to the outside. You don't risk vandalism / theft. It's way easier to do. The only downside is that you don't get a good downward view if you're looking to use it to help you parallel park
Mounting it outside is good for the view. Other than that, everything else is a pain in the ass. You have to run the wire through the existing opening at the top left of the tail gate and through that tunnel on the left. If you don't, the rear window will snap the wire on it's way down. Also, once it's through the tunnel you have to cable tie it to the existing wire loom that runs along the top of the sheet metal. You will have to unscrew the 10mm bolts that hold the toyota and 4runner emblem on the back to get the wire to pass through. Then you have to drill that piece or cut that piece to allow of the wire to pass through.
I still haven't finished putting my tailgate back together, so I can take some pictures tonight when I get home.