For me, it's more about whether I have missed something in evaluating the options. Sure Barcelona vs Rome is more or less equal, but I don't have the complete information about both of them. I might say it's easier to choose after I visited both, but definitely not before.
It's also about what parts of both the cities I'd enjoy. Overall, they might be the same. But if I like some specific aspect let's say history, Rome is better. If I like diverse cuisines for instance, them Barcelona is better.
Realistically when faced with a choice between going on vacation to Barcelona or Rome, for most people it's going to come down to a question of cost. Are there direct flights? what do they costs? Are there any special deals on? Is there a huge event in one of the places that week driving up hotel prices? Do I want to attend that event? And so on. You won't be able to answer those questions without research, and unless you are properly rich the answer will make a significant difference.
Plus there is always the implicit third option of "stay home and save all that money for something else", which also has to be considered.
It's also about what parts of both the cities I'd enjoy. Overall, they might be the same. But if I like some specific aspect let's say history, Rome is better. If I like diverse cuisines for instance, them Barcelona is better.