heart lung

It is much harder to transplant just lungs as the heart gets in the way! So in most cases it will be a heart and lung transplant. If the heart taken out is healthy then that is given to someone else who is just wanting a heart. It does not go to waste.

What unavoidable factor would diminish dthe chance of success of a lung transplant, but is not a factor at all in a heart transplant

One is not "better" than the other - they will both perform exactly the same as a non transplanted heart or lungs. Neither will make the patient bionic or superhuman. A patient with heart failure would rather have a heart transplant than a lung transplant, and vice versa. It's whatever it "better" (more useful) for the patient, but neither hearts or lung transplants are "better" overall.

Refer to your doctor

The survival rate at one year after transplant was 77% for lung transplants and 64% for heart-lung transplants