Well, of course the main question is always what you want. I have to say that art is not my main focus. That said, did you check the same page about Naples? Florence has definitely a lot of things to see, but Naples is not that bad. Actually, Naples has so many things that they can't even show them, because there are not enough people. They are accessible only in the weekends of May. In Florence, on the other side, you don't have this: http://users.ugent.be/~jhvdewal/napoli.jpg and this (Capri): http://212.239.51.7/cral/images/stories/capri/capri.jpg and this (Ischia): http://operaischia.com/Images page/LighthouseFromCitara.jpg and this (Procida): http://www.procidamia.it/vivara.jpg and a couple of other things. To have that in Florence you have to travel a bit. But honestly, if you like Sulmona I doubt you would like Naples. Few cities in Italy, if any, are more chaotic. Definitely it's not a quiet place. That was just my personal preference.
A opinion about Sulmona: http://www.tripadvisor.com/ShowUserReviews-g194928-r5935013-Sulmona_Abruzzo.html
I had just came back from Italy - and Sulmona. A very nice place among the mountains, but also very cold...I will go back in the summer.