I suppose the plus side for using the Bus would be that it only takes 2 hours to get from Tortosa to the airport (a lot quicker than the trains).Although if flying from Terminal 2 then the journey time increases a bit due to the shuttle connection. Also the last bus leaves Terminal 1 at 21.45,which is later than the last train to Tortosa from Sants.The fare when buying a return is around €33 Euros,so cheaper than the train. (I believe RENFE may now only sell day returns?) Of course if you have an OAP discount card,then your train tickets will be cheaper.
It would be much safer I bet.Not having to worry about Pick pockets at Sants.Although I suppose if you fell asleep on the bus journey,its possible some herbert may be on your bus & try his or her luck?  For older folks ,you would not have to worry about getting your case in the overhead luggage rack on the train,as buses load cases etc.. under the seating areas. You need (according to the HIFE website) to purchase your ticket at least 24 hours in advance of your intended return journey from the airport.I suppose this could be awkward if you suffer any air delays etc..  At least you can just turn up at the airport train station at any time & buy your ticket & get on the next train.
So I would say maybe better to travel up by bus (you will know exactly what time your bus needs to leave & buying on line with HIFE,I suppose is straight forward)  & return by train. major train delays have in the past nearly made me miss a flight! Only quick work from a barcelona cabbie saved my bacon.