Yes always happy to help!! I understand. It’s both exciting but stressful you wanna make the best choice haha. Chanel don’t come cheap and even if it did they are items to be cherished right?
I took some photos for reference. For what it’s worth I hear the rectangle mini holds about the same just a different layout because it’s more horizontal not vertical to organize. I wanted to show what I might carry on a regular day in each bag and what fits. I have a mini (full disclosure the sizes on CCH are confusing AF! I thought I was ordering a small but think I got a mini…basically they changed the classifications but it’s whatever one was 4200 in June) CCH and a mini square. They would both hold more if I used a cardholder which is my norm but I went to a few stores where I have credit cards and wanted to take a few extra with me recently

as you can see, the only thing I couldn’t pack easy were sunglasses (foldable ray ban) so i’d just clip those on my shirt/belt or like a headband.
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The reason I think mini flaps in black are popular is they’re (or maybe were) a more approachable price point than a lot of Chanel bags and give you a traditional classic look of shiny hardware and black quilts. People also love them because they’re lighter than the Cf and are lighter because they don’t have the second flap. Many say the small CF and mini rectangle are nearly identical in capacity but I don’t know that from personal experience.
I think they likely got a new shipment due to 21k launching recently. If it’s what your heart wants I say get it! Honestly I don’t think about investments in terms of money I just think about my wardrobe ans budget. If you feel you want a versatile bag that is convenient and brings you joy, and a mini rectangle in black is that, you really can’t go wrong. These bags are beautiful and such a joy to carry that if you got the one that most excited you, you will be happy. If I started over again I would still choose to start with either medium CF or mini flap (either shape tbh) becauseI love the classic look and I knew those would be the anchors of my Chanel collection. From a mini flap you can decipher What you would like to add if any imo! Since capacity is pretty good, they have an elegant look and the size makes them easy to dress up or down, you can’t go wrong.
CCH is a fan favorite of many but mini flaps are always going up in price and desirable. I don’t think you’d regret starting there if you knew