alpine are very proud of their screens... You can get screens made by other companies for much less and better quality.
As far as portable goes, I'm not a fan. Anything and everything portable is basically a big "steal me" sign. I've had atleast 5 people the past 2 weeks come to my shop and report theft of GPS and portable DVD systems. So sure, you pay about $300 for that portable unit, but if your stuff gets stolen you end up paying a lot more!
Headrest monitors are expensive, but are also probably the most convienient of DVD solutions. Most have individual DVD players for each screen, so if you have kids that fight over what to watch, problem solved. Overheads are cheaper installed and a bigger screen... But they block your view.
In dash DVD is nice for the driver and passenger, and makes installation of an overhead cheaper because you can get a TV without DVD built in. You can then feed the DVD from the indash to the tv saving a couple hundred $$.