For me, it's more about the placement options than the player.... Placement = controllability = me being able to see what track is playing and advance it easily without taking eyes off the road.
Regardless of the player, get a Bluetooth audio connector for the aux input. That allows you to get clear sound through your speakers from anywhere around the car.
iPhone Bluetooth audio might be the perfect combo, if you can mount it somewhere. You can get a Bluetooth transmitter for almost anything these days too, allowing you to spend time solving the aforementioned placement problem/ priority
