We have a couple of systems. No particular brand recommendations but a couple of things to look for:
– make sure they have a battery. One of our systems has no battery so it restarts every time you turn the ignition. Thus I now have to start the engine then get out and start the DVD. Total pain. (May not apply if used for older kids)
– SD/USB card input is useful. Way easier to carry than a stack of DVDs. Make sure it can play DivX/XviD etc if you are obtaining your backups from the internetz.
– video in/out is useful. Allows one to act as player for both screens. Also means you can hook them up to TVs in hotels etc which can be handy.
– check how they mount on the headrest. Some of the straps are very annoying.
Tablets are an option but DVD players are pretty cheap these days. We got a two-screen system for less than the price of one Hudl.