The 5.1" P10 keeps the Leica optics of the P9, whereas the 5.5" P10 Plus has improvements to that. I'd have liked to see the improved camera in the P10 as well, rather than, as is so often the case, the smaller device having lesser specs. But both are good:
Huawei P10 + P10 Plus specs | Android Central
Huawei P10 + P10 Plus specs | Android Central