These two aren't really fighting for the same job, mate, so let me split them properly.
Farrow & Ball is the bigger, deeper box. 301 colours, and the strength is greens — 67 of them, which no one else touches — followed by blues (41), neutrals (38) and yellows (23). The LRV range runs 5 to 92, so you can go properly inky or near-white. The whole F&B thing is colour with complexity: pigments that shift through the day, that chalky Estate Emulsion finish, and a depth you feel in a room before you can name it. Something like Acid Drop is exactly the kind of zingy, characterful green you only really get from their palette. All White is their cleanest, most honest white — no underlying grey or yellow muddying it.
Sanderson is the more curated proposition — 154 colours, with the muscle in neutrals (27), greys (22), whites (22) and blues (21), LRV from 3.6 up to 89.7. It's a heritage interiors house first and a paint brand second, which is the point: the colours are built to sit alongside Sanderson fabrics, wallpapers and the Morris archive. If you're decorating around their textiles, the colours drop in beautifully — Airlane Blue and Amanpuri Red are exactly that sort of considered, soft-edged heritage tone that plays nicely with patterned schemes.
So the honest steer: if you want range, green-led schemes, and the most atmospheric finish on the market, go F&B. If you're building a coordinated room around heritage fabric or paper — or you just prefer a smaller, less overwhelming palette to choose from — Sanderson earns its place.
One practical thing: never pick either off a screen. Both brands sell sample pots and large peel-and-stick swatches — order two or three, stick them on the wall opposite your window, and live with them for two full days before you commit. Colours from both houses shift hard between morning and lamplight, and that's the whole reason you'd pay for them in the first place.