I think they’re both beautiful. Rosemary is a little more striking but long and less tailored than your others - and Julia, I think, goes really nicely with your other three but has a less obviously southern/stylised feel. I’m leaning towards Julia myself - both bc it matches so nicely but also bc I don’t love some of the ways of saying Rosemary - but it is a “can’t lose” decision, truly.
Take or leave but are some other combos that might appeal as using your honour names & complementing your three:
Louisa Kate
Katherine Julia nn Kit
Julia Louise nn Juno
Rose Mary-Louise nn Rosie
Kate Mary-Louise
Rose Katherine
Juliet Rose nn Jet or Josie
Julia Rosemary
Mary Louise “Maisie”
Edit: This was in my unsent comments - just read comment above and I agree with it. The nn’s for Julia are great (with Rosemary giving Rose, Rosie or Romy).
Good luck!