I think it will depend on the volume of your foot and how the boot wraps. I have a pair of the boots and shoes that are made on the same last- the boots fit great, the shoes stretched out and don't work for me*. In general I would "size down", but I find US sizing pretty useless anyway.
*If anyone needs a pair of broken in 8109 Oxfords in a 9D at a good price, let me know.
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