Honestly? You can't go wrong with either — both are proper paint, made to a high standard, and miles ahead of the supermarket stuff. But they're not identical, so here's how I'd split it.
Colour range. Farrow & Ball runs 301 colours with their famous strong suits in greens (67 of them) and blues (41). Little Greene sits at 250 colours but stretches further tonally — their LRV range runs from 0.4 right up to 97.2, so they go darker AND lighter than F&B's 5–92. If you want a near-black or a brilliant clean white, Little Greene has more at the extremes. They're braver on bold colour too — Mister David is a strong mustard yellow with real backbone, the sort of shade F&B's curated card doesn't attempt. F&B's All White, for its part, is a neutral pure white with just a whisper of warmth — it never tips cold.
The finish. This is where F&B earns its reputation. The chalky, flat, light-eating quality of their Estate Emulsion is genuinely lovely on walls — colours like Acid Drop read with real depth. Little Greene's Intelligent Matt is tougher and more wipeable, which matters in a hallway or with kids about.
Coverage and value. Little Greene tends to cover better in two coats and works out slightly cheaper per litre. F&B can need a third coat over strong colours, so factor that in.
The 'but what about durability?' Estate Emulsion is the delicate classic; F&B's tougher, washable product is Modern Emulsion, and Little Greene's Intelligent ranges play the same practical role. For bathrooms and kitchens, Little Greene's Intelligent Eggshell and Matt have the edge for moisture resistance — something like Air Force Blue holds up beautifully in a steamy room.
My advice: don't choose the brand, choose the colour. Get tester pots of your top two or three from both, give each two coats on a big A3 board, and move it round the room morning and evening for a couple of days. The right colour beats brand loyalty every time.