Hello
The train is cheaper - especially  if you are over 60 and using a gold card.
Here are the times for you
airport 1238  1308  1338  1408
sants   1257  1327  1357  1427

sants     1503
Tortosa  1720

Buy your ticket at the airport either from the machine (that displays in english) or from the ticket office. Buy single ticket to Tortosa. Dont buy one to Sants and then another to Tortosa. They dont sell returns - only day returns
You will have plenty of time so you can have a coffee at Sants and get your bearings. The noticeboard re departures to Tortosa is at the left hand side of the booking hall. The right hand one is for the long distance services. The attendants in the station all speak english. Enjoy your trip