I think you answered your own question....."dead of winter"!!! October can be hot and humid in Hawaii-and no whales. January, especially on the BI or Maui, you'll see lots of whales, and calving season starts more in mid-late February.
Wander thru the boards for each island to get a feel for each island's pros and cons. How long will you be here? What are your interests? What's your budget? Do you prefer condos or a hotel?
As to the weather.........it's waaaay better than Mass in January!!
We honeymooned in Maui so were thinking in best to visit a different island. We know we want a few days in Oahu and the remaining time (approx. 6 days in either Kauai or the big island). We like to stay in high end luxury resorts with ocean views and a great pool and beach. We love excellent quality food and warm weather with nice breezes. We want to have a vow renewal service done there too.
If you want Hawaii weather at it's best, you probably want October. But if you want the best life experience, I'd agree with going in January. The improvement in weather would be far greater in January.
My vote would be January. I love to go somewhere warm when it is winter where I live. I have been in each season but winter is my favorite. Do not worry about weather--stay on the dry side of the island and you will be in paradise.
Forget October, it's Ironman time here and this place is crazy for two weeks!
I've been in KVR in January (nice) February (nice) May (nice) June (nice) August (nice) September (nice) and October (nice). Coming from climates with seasons, Hawai'i seems season-less. Kona Village gets only about 10 inches of rain per year, but when a storm comes through, it rains. And I've been there when it rained a lot for several days. C'est la vie. You'll find mid summer to be warmer (slightly) than min-winter. To me those are like normal day-to-day variations in a climate with seasons.
There is nothing like laying on the beach when you know home is snowbound! :-) Just know the ocean conditions will probably be rougher in January than October. The weather on the Kona coast is probably going to be better than Kauai or Oahu. We went to Kauai & the BI in December 2008 and Kauai was horrible, rainy/lightening storms. Our condo grounds were flooded. (of course that's not the norm there, it was way worse than usual) Three days of that and then we got to Kona and it was sunny and gorgeous.