My. 02, go with headrest mounts. With the monitor down, you'll feel like you're driving a box truck; there's no visibility via the center rear view. It also creates an uncomfortable viewing angle for the rear seat passengers, kind of like sitting in the front row at a movie theater. My sister/b-i-l wish they'd gone headrest instead of center mount with their Expedition...live and learn.
If you're planning video for kid entertainment, I'd also suggest considering a portable unit. That's the route we went since we choose the road trip vehicle based on the needs of the trip. One DVD player serves both vehicles. Of couse, we're also probably the exception in that we don't even bother with the player unless the trips 2 hours (one way) or more. As it is, my 8 year old spent more time playing DS than watching DVD on our last two trips.
-Brent
