Maybe I did something wrong but pre-pandemic, the breakfast at Duchess seemed to have the same spread and menu as at Mr. Porter (I.e., there would be no difference).
The (for me) significant upside was that it was usually much quieter (so much more calming that I would usually go there no matter which side I was staying on).
But I let others comment if there was indeed a difference that I was not aware of until now (I am not saying there wasn’t, I just haven’t been aware).