I've been to Barcelona and I'd say there is loads there to keep you and your boys entertained. It has both great architecture like the Sagrada Familia and Park Guell and other stuff like the aquarium and football stadium. I think you'll all love it.
It's easy to get around on the public transport system - you can buy travel cards that cover you for so many days and then you just get on and off as much as you like.
We stayed 5 days (ok one of them was spent at the F1 circuit) but we still didn't get to see everything so we'll go back at some point.

Hope that helps.

Dawn

I've been to both Barcelona and Venice . I Think you may run out of thingss to do with the boys in Venice where barcelona I agree you could spend ages there. football stadium is very impressive. Some brilliant places to eat in both but I guess Barcelona would have more choice and be slighly cheaper. if you go to Barcelona watch your purse - we were there for 3 days and saw loads of pickockets and 1 mugging. Whichever you choose have a great time