I never lost a discus, but I lost my first ray in 12 hours, I don't claim to be an expert, but rays seem harder. And its a trick of the camera, the gar is about 6 inches above the ray :>
Depends how you define harder, rays dont take any changes well and will die if even the slightest little thing is wrong, most rays will die within the first 48 hours if you get anything wrong, they are also highly sensative to almost all aquarium treatments so if a ray gets sick then its almost certain it will die unless you take it to a specialist vets. BUT, once aclimatised and eating providing you keep the water clean and the nitrates low rays are very hardy fish and become a joy to keep where as discus seem to need almost constant pampering or they start to sulk and get sick.