depends on your needs. i have both. the boy is edgy but it fits very little (which is fine with me); i can fit more in the sq.mini but the chain from this season is just a bit shorter on me than the boy.
I have both the so black mini square and so black small boy. I'd say it depends on what you want --
IMO, the small boy is slightly more luxe and unique while the square mini is more casual and low-key. The square mini holds just a little more than the small boy and because of that, feels a bit more functional.
The strap drop on the mini is ~19.5-20.5 and the small boy is ~22-23in (if I remember correctly). I'm 5'4 and can barely wear the mini crossbody though it sits pretty high. The small boy falls perfectly crossbody on me.
I think it really depends on your needs and what you already have in your collection. Both can fit similar amounts but there is a little more leeway with the square mini due to the structure. They fit my daily essentials such as my Iphone SE, YSL card holder, LV key cles, tissues, blotting sheets, gum, earbuds, portable charger, hand sanitizer, chapstick, lipstick and sunglasses. The small boy can be worn casually and formally so it works for both day and night. You also have the option to double strap the chains and where on the shoulder if that appeals to you so in that sense, the boy is more versatile IMHO. However, I think the square mini is more practical because of the turn lock closure which makes it more easier to get in and out of the bag. HTH. Good luck deciding!