Mega Scizor's weakness is to it overheating, but I'm gonna say it has the advantage because of their natural tactics.
Scizor is neutral or resistant (Bug STAB) against all of Mega Pinsir's moves, whereas Scizor has SE Flying-type moves, & many of Pinsir's moves are detrimental to it:
Thrash will make Pinsir use only that exclusively for a while, & possibly become confused after. Submission inflicts recoil. Superpower drops ATK & DEF, Close Combat drops Physical & Special Defense....
Pinsir also has NO range outside of possibly Me First, unlike Scizor, with Vaccuum Wave, Razor Wind, Air Slash, Bug Buzz & Silver Wind.
Also, some Pokedex entries for Scizor:
Scizor:
This POKéMON's pincers, which contain steel, can crush any hard object it gets a hold of into bits.
Mega Scizor:
Due to the effects of Mega Evolution, its pincers have taken on a more diabolical form, ripping anything they pinch to shreds.
Admittedly, it's also stated its better at using them to smash than grab. But still: Unless it spams Me First and status moves, Pinsir WILL go into Close Combat, and try to pinch/imaple Scizor.
Now, what happens if Scizor decides to claw this flying pests's wings?
I'd question how well Aerilate can turn Normal-type moves into Flying-type moves if Pinsir gets its wings clipped & is unable to fly.
Scizor having access to Counter could also be rough on Pinsir.
Iron Defense, Swords Dance & Agility are also useful. (Although, Storm Throw can counter Iron Defense.)
And yes, Scizor gradually overheats because of its stored Mega Evolution energy... but I doubt that's fun for Pinsir to try & grip either.
One thing about Pinsir is it has Seismic Toss, which our wiki claims: "throws the opponent high into the Earth's atmosphere, to come crashing down." Supposing Scizor can't return (I doubt it. Gravity is a thing.) that might be a means of victory.
Also, Guillotine, but that's done by going for the head; Mega Scizor is a whole foot/0.3 meters taller than Mega Pinsir, might also be able to fly, and probably isn't going to let Mega Pinsir pinch its head, given its huge pincers of its own and how head on the fight would be.
So yeah. Voting Scizor, High Difficulty for range, more flexible statistics manipulation, less moves that aren't detrimental, actual Super-Effective moves, the fact that touching overheating Mega Scizor probably hurts Pinsir, too.
...And the fact that Mega Pinsir's huge wings not only seem necessary for Aerilate, but are easy targets to pinch off (Which Scizor should be able to do easily.) in the close combat it'll try to enter, whereas Scizor's wings probably aren't as easy to access with Pinsir's pincers.
