Just from my personal experience... I vowed not to buy another classic speedy 25 since I sold mine last time, it looked right size on me but a bit old fashioned (30 looks casual and chic on me even my hubby says he likes the size 25 on me, I don't too trust his fashion sense by the way), plus the opening is so small, but I may buy a speed B in size 25 because for a cross body option the 25 size looks the best on me than the 30, still can't decide to go mono or ebene, as for now, more towards ebene.
but still, best to try it on before making the purchase, you should buy what you love.