If you have enough time to use 3 Overheats, Overheat is clearly better.
If not the difference between them is unclear. Longer battles favor overheat more. Considering Enteis good durability i would probably go for Overheat.
Congrats on your 100%, i have 3 93%'s and a 95% that only has 13 attack. Can't quite find that perfect one which is annoying.