Evolve Onix or hold out for Scyther?

I would check that nest out first and see if you can come up with a good Scyther to evolve

I decided on a Steelix for my first metal coat. Level 20, CP 1345, Dragon Tail & Crunch. He is great for prestiging. Especially nice against Snorlax, as he takes reduced damage from its best moves.

Overall, Scizor is the better option in terms of gym usage. Steelix seems to be a good defensive pokemon, but his max CP is too low and he'll be kicked out of gyms pretty easily, as an attacker he's simply not that great. On the other hand Scizor is an excellent attacker.

Having said that, you have no garantee that you'll find a good scyter to evolve in your visit to the nest, so there's that to consider. If you have the option, I'd evolve Scizor, but if you don't have the prospect of finding a good Scyter to evolve in the forseable future, then just evolve Steelix, sooner or later you'll find a Scizor and another metal coating.

You'll get more metal coats. I've gotten three already. Go ahead and evolve the Onix, but still farm the Scyther nest as much as you can. Set a side a couple of good Scythers, and rack up as many candies as you can, and you'll be ablr to get your Scizor soon enough.

I evolved a Steelix first, got two steel moves, and have found it to be useful for prestiging against Lapras.