trying to decide between Bali/Tulamben or North Sulawesi/Bunaken for 30 days in either June or July.
For 30 days you will be better off with North Sulawesi, more dive sites and less traveling to the sites and better diving. You can also fly in to Manado directly non-stop from Singapore on Silk Air a subsidiary of Singapore Airlines.
You will find dive operations and accommodation that vary from very basic (homestays, local food) to very fancy with air con and hot water and international menu So just find your level of comfort.
Also find out the dive operator's policies on length of the dive (some may restrict to 60 minutes, most dives that I have done range from 75 to 85 minutes) and the diver to guide ratio.
Guiding by experienced (some with many thousands of dives) local guides makes a great dive site better. Tell you guides what you want to see and they make their best effort to find it for you. Ideally diver to guide ratio should be low, 3 or 4 divers per guide is good.
If you are photographer you may want to see if you can get your own guide. Tip the guides generously. They provide a great service and their regular wages are not very much.
If you want to be in North Sulawesi in June July you should probably
book as soon as possible as that period is
very busy.
Many dive operators now have operations on the Manado/Bunaken side as well as the Lembeh Straits side. Or they have related operations (e.g. one dive operator providing diving for two different resorts in the two areas). Another factor to consider is the location of the dive resort in relation to the dive sites and how long a boat trip to the dive sites and whether the boat stays out all day or comes back to shore after one or two dives.
In North Sulawesi Two Fish Divers
Dive Manado/Bunaken and Dive Lembeh with Two Fish Divers Bunaken and Bastianos Bunaken
Diving Bunaken Diving Manado - Bastianos Diving Resort, North Sulawesi - Indonesia Bastianos Lembeh
Diving Lembeh Straits. Diving with Bastianos Lembeh Diving Resort : Manado, North Sulawesi, Indonesia both have clean basic accommodations and good dive guides, friendly staff and much lower costs than the fancier places and both have Lembeh Island and Bunaken Island resorts. And both are within 5 to 20 minutes of most dive sites.
With 30 days I would suggest splitting your time equally between Bunaken Island and Lembeh. By going with an operation that has resorts in both areas you could easily go back and forth based on when there may be space available. You wouldn't do the back and forth a lot but it gives you flexibility in your booking. (e.g. last year I did 5 days Bunaken, 15 days Lembeh and then back for 5 days Bunaken)
Do as many night dives as you can in Lembeh, they are great fun and shallow, you can see all kinds of stuff out at night on the bottom. Big nudibranchs (eight inch long
Spanish Dancers) and their cousins the
pleurobranchs. I saw a crab spawning one night. Lots of surprises. Big five foot
octopus out hunting.
False Fire Urchins racing across the bottom, they move fast at night and you will see baby
Bangaii cardinalfish Pterapogon kauderni, one of my favorites, hiding among the urchin spines
I highly recommend for the North Sulawesi area Reef Fish Identification - Tropical Pacific (Paperback) by Gerald Allen, Roger Steene, Paul Humann, Ned DeLoach. You will see many photos taken in the Lembeh Straits.
Don't forget to put aside money for your international departure tax (varies by airport) payable in Indonesian rupiah cash just after check in.
Most resorts have the dive operator on premises. A few will let your stay at a nearby homestay and dive with them if they have space on the boat. But that is less likely in time period that you are looking at (June/July). You are probably better off staying where you dive unless money is a major issue.
Whatever you do, you will have a wonderful time. The local people are very friendly, always laughing and singing.