Honestly, you can't go wrong with either — both are proper premium British paints. But they pull in slightly different directions, so let me sort out which suits you.
Little Greene is the broader, more versatile range with 251 colours. Its strongest families are greens (48), neutrals (44), blues (28) and whites (22), and the LRV range runs from a near-black 0.4 right up to 98. That spread means you can do almost any room — soft chalky neutrals through to genuinely deep, characterful colour. The blues are a particular strength: Air Force Blue is a cracking confident mid-blue that holds its own in a hallway or study without going navy. Mister David is a lovely soft sage green, the kind of restful colour that's everywhere right now for good reason.
Paint & Paper Library is the more curated proposition with 186 colours, built around its clever tonal system — colours designed to layer in graduated shades so you can run ceiling, walls and woodwork in the same hue at different intensities. Its strengths are greens (34), neutrals (33), whites (31) and greys (24), with LRVs from 3.7 to 99.2. The greyed architectural neutrals are where it shines — Slate IV and Lead II are exactly that understated, slightly inky London-townhouse look that the brand does better than anyone.
So: if you want range, depth and standout heritage colour, Little Greene. If you want a tightly edited, sophisticated neutral and grey scheme with that tonal-layering trick, Paint & Paper Library.
The "but what about quality" question — both apply beautifully, both have excellent intermix systems, both are well worth the money over trade emulsion for the rooms you'll actually look at. Get sample pots of two or three from each and live with them on the wall for a couple of days before you commit. Light changes everything.