If I had only a week and it was my first time in Bali, I wouldn't split it up and waste a day changing resorts. For a first trip, I would choose Amed or Tulamben. Both have a good mix of shore dive sites, from coral gardens, a few wrecks, walls, and sandy muck. Padang Bai is more open water focused, though there is macro and wall dives too. PB is generally much colder than Amed/Tulamben, with 23-26° the norm vs. 28 for the others. Amed is bigger so has much more restaurants, some music and night life and spas if this is important to you. Tulamben is very small. Amed also has a long sandy beach for relaxing and snorkeling, which Tulamben doesn't have. I enjoy both places but spend more time in Amed, because of the addition topside services and the beach.
In Tulamben, I second the recco for Villa Utama, and add in Wonder Dive just down the road. In Amed, I recommend Dream Divers Bali and staying at Enak Bungalows next door. Both great locally owned business, as is Wonder Dive.