As a teacher in a secondary school here in Roquetes I have to agree with the opinion that if you have kids in school then you ought to try to learn Catalan not Spanish.  It will help them a great deal more if you do.

Apart from that I think it depends what sort of person you are, as has been mentioned.  However when I say this I say it from the point of view that if you don't get out much locally and don't want to spend time in the local bar or at the local football club but do want to travel in the rest of spain then you ought to learn Spanish as it will be more useful.  But if you are local orientated and especially if you live in a small village and want to talk to the people living around you then go for catalan.

All locals can speak spanish the only ones who say they can't are catalan nationalists who just refuse to, however at any social occasion they will promise to speak in Spanish cos you are there and then switch back into Catalan after only two minutes of using spanish.  Here in the Baix Ebre Catalan IS the local language, much more than in Tarragona or up North.