I don't know your operation or your location. In the mid-west hay is very time sensitive crop. Reliable equipment is a must to put up hay before the weather turns against you and damages the crop.
If break downs on your old wagon start to cost you some crop loss, then a more reliable machine might be justified by reduced crop loss, even if it only has the same capacity as the old machine.
If your old wagon requires a lot of maintenance in the off-season, but it is always reliable enough when you need it. Then reduced maintenance and repair costs would be have to be high to justify the newer machine.