Tough question but I will try to give you a bit of info. Long Q-T syndrome can be acquired or congenital. Acquired long QT syndrome is caused by many medications. You did not mention any meds that the cardiologist was concerned about so it may be that going forward you need to make sure any Dr. who prescribes medication for you needs to know you have a long QT interval. A normal QTc < 0.40 sec.
Who to believe - your PCP or your Cardiologist. Well your cardiologist has completed a fellowship in cardiology. In order to become board-certified as a cardiologist, you must do 24 months of clinical cardiology and 12 months of elective. Therefore your cardiologist has more education as well as experiencing in dealing with cardiac disease. However that does not make him perfect.
If you are still uncomfortable I would suggest you seek out the opinion of a 2nd cardiologist. Thousands of PCPs are excellent in dealing with cardiac disease but in this case you have conflicting info.
Good luck and do keep us posted.