Iron lockbox is likely the less desirable box however, as all original accessories are in it, good and bad. The percentages for desirable things are often in the lower percentages, so you'd have to be lucky on any box you open.

For example, I unboxed a heavy wolver tail from a copper lockbox. Afterwards, I unboxed a heavy headband. Those are on complete opposite sides of the spectrum in terms of pricing, but both came from the same box. I'd recommend looking at the market's demand for accessories (auras, tails, and wings will always sell good) then find a lockbox with the higher consistency of those items. The copper lockbox had quite a few nice things in it in all honesty, flowers, nice auras (supposedly), glow eyes, tails, and a few other neat things. But it only had one new accessory, so that's a thing.