personally i would keep the snug pair.. i think they will stretch.... you can always have them professionally stretched or sock trick them. shoes that become too big are more of a pain to pad and make fit IMO
Going against the crowd here -- if it were me, I'd keep the pair that fits perfectly. I have a couple of pairs of kid Simples, including a plain black pair of 100mms that I have absolutely worn the hell out of, and they haven't stretched much at all.
My nude patent Simple 100s stretched more than my kid Simples have. That pair fit perfectly out of the box, but over time got a tiny bit loose. I stuck a thin heel grip in (Foot Petals Heavenly Heels) and it solved the problem, and they're perfect again and very comfortable.
I have a pair of 39.5 and a pair of 39. You and I are the same size and sometimes I mix them and cannot tell the difference. Over time they will stretch but if it is a 40 for you, I would say go with the 39.5.