The two have numerous similarities: they both cater to a conference/business crowd, neither is near city center, both are near the subway and a fairly quick ride to the center of Barcelona, both are near a mall, both have rooftop pools, both have views of La Sagrada Familia.
On the other hand, the Fira is definitely more stylish and higher end than the AC. The AC is nearer to the beach, and the Fira is super convenient to the airport. There are more choices for eating near the AC, but both are a fairly short ride into the city center by taxi or subway.
It sounds like your trip may be leisure and not business, so I think perhaps you'll enjoy the Fira more. If you are going this summer during the busy season, both might be better choices than crowded city center hotels. Have fun!