Depends on what you want. Fire Blast is a 110 base power move with 85% accuracy, whereas Flamethrower is a 90 base power move with 100% accuracy. Fire Blast is able to get more OHKO's, for example Aegislash, but Flamethrower is more reliable. So it depends on whether or not you want to take a chance with Fire Blasts accuracy. I personally run Fire Blast because I don't think 85% is too bad, but it has missed at important times. In short, it's a simple case of power vs accuracy.