It would really depend on the cities you want to visit - that will help you make your decision
Viva Italia visits Rome, Tuscany / Florence and Venice (this is an 8nt tour)
Bella Italia visits Florence (with side trip to Siena) and Sorrento (with day trips to Pompeii, Capri and Amalfi) (this is a 7nt tour)
Personally if it was a first trip to Italy I would choose Viva Italia because Venice is included. I have been a few times and it is one of my favorite places to visit
Pardon my ignorance, but what is great about Venice (besides the obvious)