Obviously both are great bags so I can see why you're having a tough decision!
So, I currently own both: Small Sangle in black & Alma BB in black epi leather.
I got the Celine Sangle first because I was on a Celine high 3 months ago haha. I unexpectedly fell in love with it after trying it on a whim. It was the perfect small tote/crossbody for me at 4'10'. The black looked so chic & I love how smooshy the leather feels while still maintaining its shape due to the construction & size. It's great as a casual, everyday bag due to capacity while being versatile for work if you don't have to carry a big old laptop. I wore it almost everyday for a whole month!
Now, the Alma BB in black epi has been on my mind for a while but the long strap really held me back for a long time. However, with the rumored price increase (again!), I searched the internet for a matching adjustable strap. I found a black nylon Prada strap on The Real Real with matching shiny silver hardware. I put the 2 together and I am so in LOVE! It gives the Alma BB an edgier look because of the chunky strap. I also think it looks sooo cute worn super short on my shoulder. I bought the bag just about a week ago & can't stop wearing it - both for going out a! Of course, the Alma BB works great in a casual style and a formal setting. I will say that I only like the black and the rose ballerine for the Alma BB, I do not care for the other colors they currently offer.
All in all, I recommend the Alma BB in
black epi first due to the rumored price increase and if the long strap works for you. I haven't heard of a price increase for Celine & I don't think they increase their prices as often, so you can get the Small Sangle later.
Some pictures for fun:
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