Common wisdom often seems to suggest Kauai. It's a nice and fairly rural place, but dang is it small. Given the size, and the small roads, you do end up scrambling for parking a lot on Kauai, I thought. Plus, as a Humboldt neighbor, I doubt you'd be overly impressed with how undeveloped Kauai is...compared to Humboldt it's DisneyLand
I would definitely recommend the Big Island. It has lots of variety and a ton of space. Personally, I thought the Kona area was more laid back and less busy than, for example, Kapa'a. Plus, the diving opportunities on the Big Island are AWESOME. Way more than Kauai.
So, being a country guy myself, and knowing where you're coming from, I'd recommend the Big Island in a heartbeat. I've only been to those two, but I already have my next Big Island trip planned. Maybe the only thing I liked more about Kauai was the beautiful eroded mountainous terrain. The Big Island is much younger and doesn't have the crazy topography. But it does have a lot of room, is almost all rural, and has great diving at quiet little locations where you can have the whole place to yourself sometimes. Parking wasn't a problem, either, where we went.
PM if you want some specifics. Whichever island you go to, get the Wizard Publications guidebook for that island. Awesome books. There's a Big Island snorkeling guide that is handy too.
Aloha!
Gregg