Of these two you have listed I would opt for the Rocky Mountain over the Liv. The Liv is 27.5 while the RC Instinct runs 29/27+ so to me having multiple wheel/tire size options is a huge win. The RC has slightly better fork. Also, I'm not a fan of Giant's proprietary components. I've actually never ridden or looked closely at a Rocky Mountain in person so I can't speak to that. My wife has a Liv full suspension bike. It serves her well but its heavy.
If your open to something else for the same price, I would consider the Juliana Joplin or Furtado. Santa Cruz makes great bikes and have an amazing warranty. Both these bikes have a solid build spec (no proprietary components). The Furtado has clearance for 2.8 tires. Pinkbike just posted a review also of the Tallboy (Joplin) which was great. Hope this helps.
https://www.julianabicycles.com/en-US/furtado
https://www.julianabicycles.com/en-US/joplin