i prefer the Small!
i recently got a Small classic and i've owned my mini rectangular for a couple months already. initially i wanted the M/L classic since i watched the same reviews as you that say the Small can hold almost the same as a mini.
when i got the Small, i really don't understand how most everyone can say that. my mini can barely hold my cardholder, airpods, keys, phone and a flat sanitizer without bulging a little. with my small i can fit all that plus a (very) small water bottle, lipstick, reusable bag, and sunglasses resting on top, and the bag doesn't even bulge. so capacity wise, i think it all depends on the shape of the things you have, and there is a significant difference bet small vs mini if that matters.
for shoulder vs crossbody, i tend to wear crossbody bags mostly in the winter since it's less uncomfortable on my chest due to jackets and coats. ever since i got the small i've been wearing it a lot this summer on the shoulder either doubled up or single and i love the look of it. in that sense i think the Small is more versatile.
other than that, if you feel like you'll always want a classic flap, then you should aim for a Small before the next price increase
if you also wanted caviar from the boutique, there are new colors coming out every season and recently they've been bringing back previous colors as well. my small is in caviar and my mini in lambskin, and although i love the luxuriousness of lambskin on my mini, a good season caviar can look just as good and has the added benefit of being more durable!