both make good trailers. Are you looking to get a dry van or reefer?
One thing to look for on a dry box is to be sure none of the wood is rotting on the floor.
If you are buying a reefer unit, take a look at the coolant, oil, and battery condition.
For either, look the trailer over closely for any signs of leaks, look at the wheel seals, the condition of the brakes, make sure the axles are straight (not tracking), and that the axles slide properly.
Inspect all the air lines, and if it is an air ride trailer, be sure to check over the air bags well for signs of leaks and/or deterioration.
Last but not least, look at the seals on the doors, especially on a reefer unit, and be sure the doors open and close properly.