Most direct train from Porto to Lisbon Rossio station

If you go Campanhã to Lisbon, you can catch a train at the São Bento station in Porto. It is a few minutes train ride from São Bento to Campanhã and you don't buy an extra ticket. It is confusing but easy to figure out. We stayed a 10 minute walk from São Bento and unless you are staying near Campanhã you would take the train to São Bento. The train to Lisbon is nonstop and you have choices of which station to travel to in Lisbon, as well as metros that connect.

As JA says, you'll need to get from São Bento station in Porto to the larger Campanhã station that the fast trains use.

In Lisbon, you'll probably be better off taking a metro to Rossio. Take a train from Porto to Lisbon Santa Apolonia station and a metro from there to Restauradores (there may also be a bus that gets you closer to your lodgings). Use Google Maps to find the best public transit routing in Lisbon to your lodgings.

Rossio station is a commuter train station that doesn't connect really well to the high speed trains in Portugal (but great for going to Sintra by direct train).

As I understand it, Rossio station is only used for Lisbon-Sintra services, and other lines don't connect conveniently to it.