Should I max out Scizor or Pinsir?

Scizor is the better one (resistances). Fury Cutter-X-Scissor is a fun because fast and efficient moveset.
Pinsir has Bug Bite -X-Scissor I think, also a good moveset.

Pinsir has the best bug DPS, but Scizor is super close and will typically survive longer. I like Pinsir and Mega Pinsir better but I admit Scizor is more useful currently.

Speaking of Bug types, X-Scissor and Megahorn need significant buffs and Fury Cutter should get a weather boost. I know Niantic has trouble fixing bugs, but these Bugs should be easy to fix!

I like Pinsir more but it is just from nostalgic reason alone..
Love it when battling Exeggutor in old gym using Pinsir...

Scizor has psychic resistance and only slightly lower DPS. Go for that one.

I love my level 40 Scizor. He is one of my favorites to use. That being said, Syther candies were much easier for me to get and I still haven't really found a good Pinsir.

They are both good, and come out ahead in different matchups. I think both are worth maxing and you should start with whichever one has better IVs.

Scizor gets more love, overall, for being better against Mewtwo. But you'll likely want both of them at or over level 33 if Mewtwo has Focus Blast (I seem to recall one having the final break point at 32.5 and the other having it at 33).