For Sandslash both moves are extremely close to each other, within 1% difference. I will quote my comment on Arcanine's page regarding Flamethrower vs Fire Blast:
"It is not as simple as that. Using Flamethrower twice takes 5,8 seconds while using Fire Blast once only takes 4,1 seconds. You are not taking into account that spare 1,7 seconds you have left to use your quickmove. In that 1,7 seconds you theoretically deal 25,3 damage with Fire Fang and gain 16,18 energy so you'll be able to use Fire Blast earlier the second time than you could use Flamethrower. This results in Fire Blast doing almost 10% more damage in longer time period."
^ This might give you an idea why are certain moves dealing more DPS theoretically. You can also use this sheet as a general guideline: https://docs.google.com/spreadsheets/d/1hcFo7-UGWx1k1u1BHOvDhq8foPeRr7YbX2jLjjJK0Qw/edit#gid=1036598933
