The birds are the word

I would say Sky Attack as it’s not usually obtainable and is as viable as Overheat in lots of situations. Plus, good/passable Fire attacker can be caught at high leves in the right weather.

https://pokemongo.gamepress.gg/god-bird-sky-attack-moltres-and-its-place-meta

Sky Attack does better neutral DPS than Overheat and is two-bar.

There was also just a discussion about it with good replies: https://pokemongo.gamepress.gg/q-a/moltres-fire-ou-legacy

Keep Sky Attack. It's legacy and in less risk of being outclassed by gen 4.

Keep it buddy. Moltres with SA is a phenomenal generalist and one of the best flying-type attackers. Against bug- and grass-types, it is Targaryenlike.

Meanwhile, Overheat Moltres has strong competition from Entei, BB Charizard, and, soon, Heatran.

Keep it. It performs about as well as Overheat (in some cases better) and there's much less competition on the flying type front than there is on the fire one.

I would absolutely keep Sky Attack. Heatran is coming next gen anyway, and it will be a superior choice for Fire.