JFK and LGA are both about the same selection-wise. It really depends on how you hit it.

The way UC works is that you can book at least a midsize (intermediate) car (meaning anything except an economy or compact car) and then pick anything from the UC section. Note though that if you are just a regular Gold member, then you cannot pick from the PC or Five Star sections. Those are reserved for members who have achieved those status levels. Not sure what would happen if you tried. You'd likely either be sent back to the section you were supposed to choose from or charged an upgrade fee.