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?
Apparently the Oscar last by Carmina does, as does the Olfe by Meermin that Jason mentioned. I'll be able to report back when my boots get here.