Just have a look at the stats:
Flareon: 246 Atk 204 Def 130 Sta
Entei: 235 Atk 176 Def 230 Sta
That means Flareon is the glass cannon variant of Entei.
Flareon has slightly more DPS while Entei has a better power (damage output before it faints).
In full raids Flareon might give you that extra damage that will net you an additional ball, but if you do raids with fewer people instead your Entei will last much longer and you will get much more extra balls for defeating the boss with fewer people.
Edit: Note that Entei has less defense but more stamina. This could become an advantage because it gets hit marginally harder and gains marginally more energy in effect. So a scenario where Entei is able to just fire one more charge move is conceivable and it could be that Entei deals more damage in this scenario even though it has less attack.
