Honestly, you can't go wrong with either — both are serious British paint houses with proper depth of colour and excellent coverage. But they pull in slightly different directions, and that's what should decide it.
Little Greene runs 250 colours, and its real strength is greens — 48 of them, more than any other family. It's the brand to reach for if you want historically-grounded, characterful colour with a bit of grit. The greens have proper complexity, and shades like Air Force Blue show off that confident, slightly muddy heritage palette beautifully. Mister David — a strong mustard yellow — shows how boldly they'll push colour when the room calls for it. The finish leans a touch chalkier and more matt, which flatters period rooms and uneven old plaster.
Mylands has 211 colours and its centre of gravity is neutrals (40) and greys (25). This is the brand for sophisticated, understated schemes — think calm London townhouse rather than country rectory. Mylands paint rolls out exceptionally smoothly with a slightly buttery, self-levelling quality that decorators love on woodwork and large flat walls. Acanthus Leaf No.12, an easy off-white, and Alderman No.60, a light warm putty, show how finely judged that pale end is — and when Mylands does go dark, a shade like Market Green No.38, a smoky deep blue-green, holds a refined, almost velvety quality rather than going flat.
The "but what about durability" question: both are genuinely hard-wearing in their washable finishes (Little Greene Intelligent Matt, Mylands Marble Matt Emulsion), so neither wins there for a normal living space.
If your scheme is green-led, period, or you want maximum character — Little Greene. If it's neutral, grey, or you're after that smooth, polished modern-classic look — Mylands.
My practical advice: order sample pots from both, give each two coats on a big board, prop it against the wall that gets your worst light, and look at it morning and evening. The brand matters less than how the actual colour behaves in your room. Whichever you pick, use the matching primer or undercoat from the same system and you'll get the truest depth of colour.