Either one would be a treasure. I think you have to decide if you want a small tote or a larger one. Can you see these in person before you decide? Is this your first Birkin? If it was my first, I would choose the 30. Hope this helps!
Do you carry a lot or minimal enough for a mini? & are you more of a gold or silver/white gold type of gal? This is something that you need to consider as in the end you will be the one rocking the bag
If its me, I would go with B30 in ghw since it works more with my outfits & accessories.