What should i evolve with a metal coat?

It's just Scizor and Steelix. I'm gonna do Scizor first

I think that's it. I did Scizor first.

None.

I would say scizzor for actual usefulness.
Steelix if you really like him.

https://pokemongo.gamepress.gg/gen-2-evolutions-guide

I vote Scizor

Well.. lets go like this
Steelix - great defender, steel/ground covers shitload of resistances, 2nd highest def in the game (i think) behind.. Shuckle xD
Scizor - great attacker, if you get bullet punch/iron head any rock/ice/fairy will get scizor'd
Also if you got a shitty scyther and a good onix the choice is easy

Don't agonize too much over the choice, as you will probably get another metal coat soon enough. I evolved a Steelix first, because my 100% IV Scyther has been stuck in a gym for over a week. Today, I got my second metal coat, so now I will wait for the Scyther to be freed.

My limited experience with the two mons so far:

1. Used the Steelix (CP 1432) to prestige against a CP 2200 Lapras, and it did about as well as anything else against a Lapras, almost but not quite finishing the job singlehandedly with only dodging charge moves.
2. Prestiged against a CP 2300 Scizor with my CP 1473 Magmar, and the Magmar crushed the Scizor. It was the easiest prestiging battle I've had since the move stat changes. The first Fire Blast instantly wiped out about half the Scizor's total HP.

So, if you get a Scizor, keep him away from fire.