Both are good.
MEga Manectric's tier, UU, where most common grass types are Mega Sceptile, Celebi. They can be easily countered by HP Ice. But Overheat hits many Pokemon hard. It gives you coverage to Mega Beedrill, Breloom, Metagross etc. But flamethrower doesn't lower Manectric's attack. And it doesn't hit much hard. Overheat+Volt Switch is so common Manectric. If you want a fast sweeper in your team then you should use overheat. Because it's power is 130. Also you can easily slow down their physical sweeper using Overheat+Volt Switch combo+Intimidate.
I always use Overheat on Mega-Manectric.
136 (Manectric's base SpA)×-2 attack(0.66)=89.76
If it's nature is Modest =about 98.736