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 251 colours but stretches further tonally — their LRV range runs from 0.4 right up to 98, 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. Look at Little Greene::Mister David for a soft off-white that's got a bit more warmth than Farrow & Ball::All White, which can feel slightly clinical in a north-facing room.
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 Farrow & Ball::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?' F&B's modern Estate Emulsion and Little Greene's Intelligent ranges are both fine on walls. For bathrooms and kitchens, Little Greene's Intelligent Eggshell and Matt have the edge for moisture resistance — something like Little Greene::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, paint big swatches, and live with them for a couple of days through changing light. The right colour beats brand loyalty every time.