I was also hesitating between Valencia and Barcelona. I ended up choosing Barcelona because there would be more job opportunities for my SO who doesn't work remotely.

Valencia is much cheaper than Barcelona but doesn't have a big airport nor as many good restaurants.

As for the best and worst things, I think they're valid for both Barcelona and Valencia:

The best thing is actually a combination of many: excellent weather conditions, relatively cheap cost of life, great food, access to the beach, overall good quality of life.

The worst things are the noise (cars and people) and communication.