It'd depend on your tier.
Magmortar for RU, Arcanine for UU. If this is in game, Arcanine.
Also, Magmortar is usually used a special sweeper, but it has mediocre speed. Arcanine can be used either as a special or physical sweeper, has better speed and more rounded stats overall.
Arcanine also has more useful abilities: Flash Fire or Intimidate; Magmortar only has Flame Body.