This makes total sense. And as the trains originate at Santa Apolonia, downtown, you're more likely to get the seat you need! Oriente is the second stop in Lisbon. Logical one to go to, if you're coming straight from the airport, but if you need to go downtown to the cathedral for your credencial anyway, Santa Apolonia makes more sense.
Also, in the Santa Apolonia station itself, on the right-hand side as you go through from the main entrance where they sell tickets, down towards the end of the building there's a small but complete Pingo Doce supermarket. You can buy all the snacks, bread, cheese, etc. etc. that you might want for your trip north to Porto right there. The food on the trains is edible, but a picnic is way better and cheaper! (And when I say "small but complete" this really
is a complete supermarket. Has its own bakery--this is where you need to meet real Portuguese bread
for the first time!)