hello
Particle effects will give you the best looking flames, but it is also very demanding on the GPU.
Sprite animation might not look as great, but it wont tax the GPU much .. perfect for mobile platform.
You can also use Particle effects with your sprite animation, wich will give you the best of both worlds ... instead of using a regular texture for your particle, you use your texture sheet (sprite animation).
So, it all depends on what platform and what kind of computer power you are targeting.