Darmanitan without a doubt
Here are my personal reasons:
- Darmanitan may have a weaker base stat total, but Flareon's potential is wasted on Special Defense, which is dumb because most powerful Rock and Ground moves are physical attacks, and will dominate Flareon's poor defense and speed
- Darmanitan learns some of the most powerful moves in the game at very early levels, such as Thrash, Flare Blitz and Superpower. This way, you have a lot of options for powerful and potentially OHKO moves. Flareon's most powerful moves are Flare Blitz and Last Resort, and these are hardly useful, with Last Resort's lack of coverage or STAB, and Flare Blitz's recoil does not pair well with Flareon's base 65 HP.
- Darmanitan's dream world ability allows it to switch forms when it takes a lot of damage, causing a massive boost in Defense and both Special stats. This allows it to tank damage while you heal it and makes good use of the move Overheat. Flareon's poor defense and lack of any kind of reliable recovery move mean it will more than likely be OHKOed, and its ability, Guts, is pretty much worthless because the only way to start with the speed boost is to give it a toxic orb. I refer to previously mentioned bad HP stat.
This is why you should choose Darmanitan. If you really want Flareon, that's fine. But you'll need to be much more careful with it in battle, and its poor speed makes it unsuitable for sweeping, wasting its above average attack.