October and November are generally good times to dive in Hawaii. For the most part, 'til the holiday week anyway, it's less crowded than some parts of the year. Our water doesn't start cooling much 'til later in November - I think we were still pushing 80-81 in early December in Kona last year.
Kona and Maui are generally considered to be the best diving. I'd listen to Wildcard's opinion on the diving as he's probably one of the few here who's done much diving on both islands. The little diving I've done on Maui has been similar to Kona, but worse viz on near shore dives, and much, much more algae than Kona has. I do like the ends of Molokini for the shark action we had when I was there, Kona really doesn't have readily accessible spots for bunches of sharks, generally a few spots to see a sleeping shark or two.
Kona has lots more good diving nearer to the main harbor than what I've experienced in Maui, while most of the Maui operators seem to make a trip to either Molokini or Lanai. Kona is usually pretty calm conditions throughout the day, except when a big swell is in, while it seems most of the Maui dive ops hit the water early and try to be in by noon to beat the tradewinds.
We had to pay more in Maui than I've seen charged in Kona, probably the length of the boat ride, or sheer volume of tourists may factor in there. I think most of the ops in Kona are still running in the 99-109 range for a daytime two tanker, while I've seen a lot of 130-140 on the trips I was doing or looking at in Maui.
If you ever do make it to Kona, and are willing to give shore diving a try, hit "2 steps" at the Place of Refuge. It's one step off the rock and you are in 5 feet of water and there's a dropoff that goes to beyond recreational depths within a 2-3 minute leisurely swim/dive. It's about as easy and as nice shore diving as you'll find anywhere in the US.
Inter-island flights should be much more available at a better price for a while. "Go" airlines just came in and they are running $39 buck one way and $59RT tickets for the summer, don't know how long that'll last, but prices will be lower overall for most of the local airlines than in recent years.
Hope this helps some.
