It is currently impossible to use an arbitary doamin object as the input to the ChoiceBox-Control because the value displayed is simply the Object#toString() value. I'd like to propose that you:
a) as a minimum allow customization of the string value through some callback
b) as the ideal solution provide the same you do for ListView because that would allow one to even show images, ... in the drop down list

For now i solved the problem by wrapping my domain object into an extra class which implement the toString