for the car (and in general, really) i like the Once Upon a Potty. it's one solid piece, so nothing to get lost or broken or anything, and it's a squat-style so they are more comfortable which means it happens faster (a big plus in the car, IMO). it also has a pretty wide base so it doesn't get tipped/spilled easily. and the front is nice and high for little boys, which is nice.
when i take ours on long trips when i know it'll be used a lot, i usually keep some disposable wet wipes handy so i can keep it clean. if you have some old towels or prefolds you could lay those inside, too, which helps keep the potty cleaner (but then means laundry to do of course).
it's small enough for infants to use, but my 5 year old still uses it in emergencies comfortably, and my very big almost 3 yo uses it pretty frequently. when we're not going somewhere i keep it in the house - we only have one bathroom and thus one toilet, so if someone else is in there when he needs to go urgently, he uses it.
and i've had it since DD1 was about 9 months old and it's still in perfect shape.
