Porto sao Bento or Campana closest to airport?

To get to the airport by rail, you should use the light rail (not the train): Trindade and Bolhão stations are close to the city center of Porto (and thus to São Bento). Campanhã is a few stops further out, in the opposite direction vs. the airport, but directly on the line.

If your question is about getting from the airport to a train station for onwards travel (or vice-versa), then Campanhã will be more convenient.

Balso I am on the train arriving in Sao Bento and will probably get a taxi to the Porto airport but wondered which train station was closer and if the cost is much different.

See the second part of my answer: Then Campanha will be a faster journey. The light rail is direct, BTW.

Balso I am on the train arriving in Sao Bento

Arriving from where? If you really want to take a taxi I'd recommend alighting at Campanhã instead. But I don't see the point in a taxi to be honest, you can take the metro direct from Campanhã to the airport for €2.

Thanks Balso and Badger, I ended up sticking with my original plan to get off at at Sao Bento and it worked out. You’re right, I could have taken the Metro. I made some slight mistakes in planning the last bit of my trip with regards to transportation.