Here is my 2 cents on both...
Pros
headrests
1. If you have multiple kids, you can get headrests with DVD players built into both that way each can watch their own movie or both can watch the same thing.
2. Can be removed and put back to factory when you sell/trade the vehicle, and you can order blank headrests for your new vehicle.
3. Doesn't block rear view mirror
Overhead
1. Bigger screen
2. Pretty cheap to do
Cons
Headrests
1. For brands like Vizualogic (who are damn near close to a factory match for cloth/leather), they are expensive unless you get them through an unauthorized source
Overhead
1. Blocks rear view mirror
2. You can remove it when you trade in/sale the vehicle but you'll have a big hole in your roof
Personally I would go for the headrests hands down, BUT at my shop I do twice as many overheads as headrests.