Hi Jon,
We have flown from both Barcelona and Valencia airport. Barcelona is a much bigger airport and there is usually more traffic about as you leave in direction of Tortosa. Valencia is much smaller and provided you don't get lost like we did first time ( having to drive through the city as a result) it is really quite a quick and easy journey and the roads are usually quieter. Basically Valenciait is further by road and you can expect to add on half an hour to get to Tortosa. We thought it was worth it though as the tricket prices to Valencia are often lower. If you can get a cheaper ticket then I would say it was worth it, if the ticket price is the same then I'd probably say stick with Barcelona as it is a shorter journey and less petrol! Easyjet fly to Valencia but I believe not all year round, ie. last flights end of October or thereabouts. There is always Reus of course!
Hope this helps!