Is qtc more important than qt?

Yes: Within normal range of heart rate, they are close but the corrected qt would be more useful. Changes in qt interval are also important to follow.

Yes, QTc: The qt interval measures the rate of myocardial repolarization. There are diseases of the heart that affect the ion channels that control the rate of repolarization and there are drugs that affect those channels. For complicated reasons, there are other factors that affect repolarization speed. "correction" for heart rate yields the qtc which "corrects" for some of these factors.