Clonazepam is stronger than diazepam but it has a shorter half-life. Clonazepam has a half-life ranging from 18-50 hours and diazepam's half-life ranges from 20-100 hours. So, on average, clonazepam leaves the body in half the time that diazepam does. This means that clonazepam has to be taken more frequently than diazepam.
I suppose we could say that clonazepam is better for short term, more severe anxiety issues, in comparison to diazepam. That being said, diazepam has been on the market for much longer and some doctors feel that it's a more reliable drug.
Doctors consider the advantages and disadvantages of a drug and prescribe it only if they feel that the advantage are greater for the patient. So it's best to trust a doctor's opinion. But if there are adverse side effects of a medication, of course that should be shared with the doctor.