I've got both also and the Simpson run slimmer, but I still wear a 9 in both. Robert is also a bit roomier in the heel. Both fit me fairly well though my Simpson are a derby not an Oxford, so probably fit a little bigger than a Simpson with closed lacing would. Hope that helps.
Does the roomier heel fit you better or cause issues, Jas?
Does anyone know of any lasts, that in their experience, boast a narrower heel and average to wider forefoot?