My vote is for a portable unit(s) that attach to your existing headrest. Pros are switchable between cars if you travel in more than your 4Runner. Also allows DVD playback at destination lodging that may not have other suitable entertainment for the children...BTDT. Less expensive!
Biggest con is potential unsightly wiring, but it depends on how you to intend to use it. We only break out the player for trips longer than 1 hour, so we're not dealing with the wires every day. Also, the Panasonic unit we bought gives real world 7 hours of run time at full screen brightness (rated for 10 or 12 on low) making the headphone the only wire in use most of the time.
2nd place would be built-in headrest units. My sister's Expedition has a roof mount drop down unit...feels like driving a box truck when the monitor is down; no rear visibility through the center of the vehicle. Also, the viewing angle for a roof mount is a little hard on the neck, IMO.
-Brent