As others have said, First is First, both in terms of service, amenities & food/beverage & no matter how good the Business... BTW I believe EY stopped using the “Diamond” tag for First, & its now First Suite, First Apartment or First Residence.
It's just The Residence, Etihad don't prefix First to it - as it's a cut above First. True, Etihad have recently removed the prefix Diamond to it's old First class, and now just uses the generic suite instead, but given the wide use of suite across many airlines and that this change is fairly recent (and thus many people still know this product as Diamond) I tend to refer to that specific Etihad moniker (it's rather unique to them). Just helps keep things distinct (given technically the Business Studio is a suite, as well as the First Apartment) - regret if that confused you.
Given the rise of some Business class seating/product, I don't think blanket rules really work so well anymore (that First is always greatly superior to Business) - take Qatar for example, where their First is more refinement than evolution over their Business product. I recommend approaching these questions on a case by case basis, taking into account what's important to you individually - especially since these decisions always involve an opportunity cost (yes First may only require a third more points than Business, but those point could have also been used towards another award booking - for example, The Residence might be a great product but I was not tempted to buy it given that I could multiple First flights instead).