These two aren't really fighting in the same weight class, mate.
Benjamin Moore is the premium pick. The Regal Select and Aura lines lay down beautifully — thick, self-levelling, brilliant coverage, often two coats where lesser paints want three. The colour library is enormous: 3,883 colours with real strength in Greens (610) and Neutrals (429), and an LRV range from 4.6 right up to 90. The pigment load shows in deep shades especially — something like Benjamin Moore::Turmeric or the warm, sunny Benjamin Moore::Yellow holds its richness without going patchy. If you care about how a colour reads on the wall, Benjamin Moore rarely disappoints.
Dulux is the sensible workhorse. It's everywhere, it's affordable, and the trade emulsion (Diamond Matt in particular) is genuinely hard-wearing and scrubbable — better in a busy hallway than people give it credit for. The range is smaller at 2,092 colours, strongest in Neutrals (335) and Blues (265), with an LRV spread of 1.2 to 92.3. There are some lovely deep tones in there too — Dulux::Night Jewels 1 and Dulux::Pharaohs Gold 2 are properly atmospheric.
The honest "but what about cost?" answer: Benjamin Moore costs roughly double, and you pay for stockist scarcity too — it's not on every corner in the UK. For a whole-house repaint on a budget, Dulux Diamond Matt will serve you well and you'll never struggle to grab another tin.
My practical steer: if it's a feature room, a deep colour, or somewhere you want that flawless finish — spend on Benjamin Moore. For ceilings, rentals, kids' rooms and large areas where value matters, Dulux is no compromise at all. And whatever you pick, get a proper sample pot up on the wall before you commit — both brands shift noticeably between daylight and lamplight.