which is better.a normal hard workout..or the sauna belt..?  

Like you, I honestly feel that a good workout plan consistently followed would perhaps yield more health benefits.

Also, I surmise that you would likely feel a greater sense of accomplishment by working out than by solely relying on a sauna belt to lose weight, since you'll more probably work harder if you opt for the former.

Sauna belt, to be sure, may perhaps be very effective for some.

However, I am not quite certain as to its negative effects, if any.

If sauna belt would be of any use, I feel that its function would only be supplemental or complementary to exercise or workout.