How much do you carry? Try to put your usual stuff in the 25 and the 30 in their regular monogram bags and see which one you find more functional. Try on both the 25 and 30 in regular and visually take in which one you prefer more. Then ask the CA for his/her advice.
I find that CAs usually give pretty solid advice because they've seen too many ppl of all ages and sizes and can categorize ppl accordingly.
I'm eyeing the 25 not because I'm 5'5 but because I really don't want a bag to overwhelm me and subconsciously make me carry more than I need to. Of course, there is a need for bigger bags. I have 2 35s in monogram. Also take in that bigger sized P9s will slouch even more.
Good luck. Which color are you planning to get? Shopping is fun and exciting.

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