In a D&D sourcebook, I want about a 90/10 mix of Rules and Lore. Odds are, if I'm shopping around for "official" 5E content, it's because I'm looking for new Rules: new monster stat blocks, new spells, etc. So I voted "more rules, less lore."
But in a non-5E D&D sourcebook, or a third-party 5E D&D book, I want about a 10/90 mix of Rules to Lore. Odds are, if I'm shopping for third-party material or non-5E material, it's because I'm looking for Lore: new factions, pantheons, kingdoms, myths, legends, adventure hooks, that sort of thing. I'm looking for shiny new stuff to hang off of the established 5E rules framework.