4 of us spent fare to visit Tarragona from Salou and then bought tickets for tarraco tren. It wasn't worth it. Just that view of mediternean sea was very good and Roman amphitheater was looking good too. But once you have seen other monuments and historical sites in Barcelona I wouldn't bothet for Tarragona. We wrapped up in 2 hours. I should have walked instead of paying 7 euros for Taracco Tren ticket.
If you are keen on visiting on all the Cathedrals, meuseums and historic sites the may be but for us Barcelona was enough.