I went through this before, because the price difference IS rather large, but I think it comes down to the different design. I noticed that although the small boy holds slightly less than the mini flap, I loved the design of the boy (thick chain with long leather piece), and all the different looks you get when you adjust the strap on it. The mini flap has a very classic and elegant look to it along with the pocket in the back which is a big plus IMO, but I still ended up going with the small boy because I just love the look of it.
I have both and both are completely different 😁 sorta like comparing apples to oranges. The classic is more feminine classy and can be worn well at night all while being lady like and the boy is just so fun youthful and edgy and good for day. Can't go wrong with either!!
Oh and just my two cents -- I've been eyeing the boy bag over the classic flap both bc I like the edgier design but also because it feels like everyone has the classic flap (and in ny, it feels like lots of people are carrying fakes), which dilutes the specialness of the flap imo. I plan on pulling the trigger on the boy as soon as bonus day rolls around 😁