FWIW, my opinion, my taste buds, etc: the stock Mazzer 151B burrs in the Duo offer an espresso that is enjoyable and is unlikely to offend anyone. Silky smooth, balanced, nuanced. Conservative, mainstream, dependable and quite forgiving when dialling in.
The SSP Espresso (HU) offer a much brighter/acidic, more intense espresso with light and ultra light beans. And it will have its lovers and its haters (“if I wanted to taste lemon I’d drink lemonade”). Add a narrower sweet spot range so a bit more faff dialling in.
The 64mm SSP Espresso delivers the same as the 83mm version. Lance and James might be able to tell the difference but I can’t.