It depends on your setup. For in game purposes, Flamethrower > Fire Blast because of the higher PP and accuracy. However it can be nullified by PP restoring items. Fire Blast is better in scenarios where damage is key and is needed above accuracy, for example in battles.
Too lazy; Didn't read: Flamethrower is good in long battles, like the Elite Four or Gym Leader matches. Fire Blast is better in link battles and battles that will be short if you can sweep.
Hope this helped!