I have VCA pieces with both these stones. They’re both beautiful and perfect for daily wear. For durability, I think onyx is more carefree than MOP, but these are also necklaces you’re considering which is different than a bracelet (more likely to get splashed with water when washing hands, clang against other stacked bracelets, etc). For a necklace that likely will just sit safely on your neck, I’d just go with the stone you like the best.
The official recommendation from VCA is that their stones not get wet. There are definitely people who wear them in showers and pools and claim they see no issues, but if you’re at all particular about your things, then I’d suggest taking the official VCA advice and not wearing it to shower. Everyone has their own threshold and may be more or less observant. What one person sees as no wear and tear, you may be bothered by. If you’re going to wear it daily but take off to shower, I’d go with either stone. For anything more than that, I’d go with the hammered gold which can be a 24/7 piece.
