David,
Sounds like a great trip.
We visited Bali in October last year and had a good time in Tulamben and Candi Dasa. (We also visited Komodo which was excellent).
In Tulamben we stayed in the Paradise Palm Beach Bungalows which were pretty nice if a little basic but very well positioned on the beach between the wreck and the drop off. The restaurant is just above the beach and one evening whilst eating dinner we were treated to the sight of a whale crossing the bay. There are other more expensive resorts there if you want a bit more luxury.
We spent 3 days at Tulamben but would have happily spent longer there as you can dive when and where you want. As well as the wreck there's a patch reef infront of the hotel which is good for critters and a very nice wall. You can also arrange to be taken further afield in a fisherman's boat. How long you should stay I guess will depend on whether you mind diving the same site more than once and whether you like the freedom of shore diving.
When in Candi Dasa we stayed at the Dasa Wana a mid level hotel set back from the main road with a nice pool and gradens (picture attached), there are quite a few of the same standard. Candi Dasa is better set up with restaurants and hotels and is more covenient for general tourism but most of the beaches there have been washed away and it's really only a string of hotels/restaurants along the main road. Whilst we were there we dived with GekoDive out of Padang Bai which might be an alternitive place to stay. It has some nice beaches and we liked the atmosphere, the accomodation though is more basic - geared more towards backpackers.
We enjoyed diving with Geko Dive they were well organised and knowledgeable and it was very relaxing to walk across the beach from their office to the boat to go diving. They do trips to all the southern dive sites - Blue Lagoon, Nusa Penida, Gilli Tepekong etc. giving quite a variety of sites. They also organised accomodation (incl. at Tulamben) and transfers for us.
We didn't get to the north but heard quite a bit about Secret Bay which isn't that far away from Pemuteran so it might be worth checking it out if you're into muck diving.
The website for Geko Dive is http://www.gekodive.com. Another operator we found helpful was Aquamarine Diving http://www.aquamarinediving.com/ they also arrange accomodation etc.
Hope this helps,
Richard