I'd recommend a portable with headrest mounting system myself...much more flexible. We went with a Panasonic that claimed 10 hours of battery life. Real world yields 6-7 with no attempt at battery saving settings like turning down screen brightness; either way, more than enough DVD time for one day. However, we only use the DVD player when taking true roadtrips, say over an hour. The portable has also come in handy if you get somewhere with no other kid friendly entertainment.
-Brent