Tribunals are designed to convict people who commit crimes in war. Although the terrorists don't represent a country, and can not "legally" declare war against us, their actions are often acts of war against a country. I believe that the courts would be an improper place to try terrorists because of this.
No, courts are not better than tribunals for trying terrorists, because what the terrorists did are acts of warfare. If the terrorists committed crimes, or harmed someone civilly, U.S. courts would be appropriate. Because the terrorists committed acts that constitute a declaration of warfare against the United States, it is appropriate to try them in the methods and systems designed to hold them accountable for what they have done.