JT, it really depends on why you are walking the Camino, I guess. Do you want to meet lots of people and have a wonderful social time, while experiencing Spain? That's what you can have from Leon->Santiago/
Camino Frances. Or are you looking for a quiet, contemplative time without seeing many pilgrims at all? That's what you could have walking from Porto->Santiago/Caminho Portuguese. We (my Caminho friends, Sebastian and Sheila) walked a great deal of time by ourselves without seeing a pilgrim soul and many times, when we did, they had walked the Frances and were on their way to Porto, to Fátima, or to Lisbon.
I loved Leon and I loved Porto. I returned to Porto after I said goodbye to Sebastian and Sheila, after we finished our respective Caminhos. There I celebrated my birthday with a walk on the beach and boardwalk, a bus tour (the best way to get to the beach, if you don't want to walk, haha!) a trip to Liveraria Lello & Irmao (most beautiful bookstore in the world, and what JK Rowlings based Hogwarts in her Harry Potter series on) and a cafe com leite at the little out of the way cafe that Rowlings wrote the first Harry Potter.
Leon has just as awesome sights and things to do--the cathedral is breathtaking.
So as I said, it depends on what you are looking for...
Perhaps you should just flip a coin!? (No matter what, you will love both places and your walk, I am sure!)