So folks...which one is more expensive?

I might add..we're going there in June/July.

I'm not sure anyone can answer such a broad question. On either, you can find budget dining, lodging, etc. I'll take a wild guess and say the Big Island in general might be less expensive than Maui, because it seems less 'touristy' to us, with more locals.
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which one has the better beaches and snorkeling?

Which one sounded more appealing to you when you researched and read Q and As and trip reports on this board?

Maui has better beaches and snorkeling.

Hawaii can be as cheap or expensive as you make it. It's more about what you do once you get there, than which island you choose.

H Bay on Oahu and K Bay on the Big Island were my favorite snorkel experiences. Molokini off Maui was a BIG disappointment.

What specifically is H Bay and K Bay? And...if Molokini is a disappointment, for a snorkel cruise on Maui- what or where would you recommend...actually, what would people recommend as the best snorkel cruise on any of the Hawaiian islands?

Why was Molokini a big disappointment? We took a sailing/snorkeling trip there once, years ago. It was beatiful clear blue water, with a hug ring-shaped coral formation just below the surface. Maybe not as many fish as some places, though. It was a fun sail out and back, with a barbecue provided on the way back. (It's been a long time, and I don't remember the name of the company, though.) It felt like more of an adventure than just going to Captain Cook's on the Big Island, which we've done a couple of times.

I think choosing where to go on Hawaii isn't about the "Best Cruise", the"Most/Least Expensive", the "Best Luau", etc. Because all of Hawaii is remarkable.

Just decide what you want to do most, how much you can spend, and make a plan. Ask more specific questions here, that people can actually answer. Like about specific places and things to do.
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According to the many travel books I've read on the islands, and from my own experience visiting them several times, I think, when comparing apples to apples, the Big Island & Kauai are less expensive than Maui.

H Bay = Hanauma Bay on Oahu

K Bay = Kealakekua Bay on the Big Island

You can spend as much or as little as you want on any island.

I agree with suze that Maui has better beaches than the BI.

We loved both Maui and the Big Island, but we felt that prices for both food and comparable lodging were more expensive on Maui than BI.

Also there is A Bay = Anaeho'omalu on the Big Island in the Waikaloa resort area. A nice beach and my son snorkeled there, but preferred K Bay for snorkeling.

I agree that beaches are better on Maui. But I think that snorkeling is better on the Big Island.

And I thought Kauai was more expensive (in general) than Maui. I felt this was because there was less competition on Maui. On the Big Island, there are both expensive and inexpensive places to eat and stay.

I've vacationed many times in Maui and two years ago I spent a week on Maui followed by a week on BI. I think everything was priced about the same (both had Safeway's with identical pricing), Coldstone was priced the same too (yum!). The only difference was that gas on the BI was about 10 cents cheaper.

In my opinion, the beaches in Maui are better. I say this mainly because there are so many that you can access pretty much any where you stay. On the BI if you stay in Kona, you will have to drive about 45 min to get to the really nice sandy beaches on the Kohola coastline.