Hi Mfed,
This may just be me, but it would be very difficult to walk in to Santiago and then have to get back to Porto the same day. If I were you I'd spend your extra night in Santiago and that way you can have margin to say proper goodbyes to Camino friends, stand in line for your Compostela, and visit the Cathedral and/or other places that others may have told you about.
If you have extra time before your Camino starts, I think walking around Porto would be a great thing then. It's a beautiful city.
What do others think?
Enjoy all the planning and Buen Camino!
Faith