So i'll try to answer.
Growlithe is a cute Pokemon who evolves into the powerful Arcanine when exposed to a Fire Stone. If you have one at your disposal, train your Growlithe until the level you want and evolve it.
Flareon is one of the weakest Eeveelutions. His Attack Stat may seem nice but his strongest physical STAB move is Fire Fang, which has very low Base power. You could use him as a team support but i don't really think that's needed in-game. Let's compare their stats.
Arcanine has higher HP than Flareon. Flareon has higher Attack than Arcanine but does not have the right physical moves to use that to his advantage. Arcanine has way more Defense than Flareon, who can hardly take a Physical hit. Arcanine has more Special Attack than Flareon. Flareon has way more Special Defense than Arcanine, letting him sponge some Special hits. Flareon has pitiful Speed, meaning Arcanine has way more.
Final Verdict
If your team has problems with Fire types, take Flareon as his Ability, Flash Fire allows him to sponge them. If your team needs a fast Sweeper with well-rounded Stats who can utilize both Special, Physical or even Mixed sets, Arcanine is the best. If you have problems with Special moves, take Flareon so he can sponge them. If you want to keep Growlithe and not evolve him.......... Take Flareon.
I would take Arcanine if i were you.
