What kind of budget do you have? St. John has wonderful snorkeling with the underwater national park & Bonaire is known for its diving.

Carrybean, we are willing to spend a little to stay at a nice place. Is St. John kid friendly? I've heard that Jost Van Dyke (as a day trip from STJ) is a piece of heaven, but I wanted to know if it's good for kids.
Also, how does anyone rate the off the beach snorkling at STJ vs. GC???
Thanks.
TomCayman, thanks for the feedback and pointing me to the kid's review.
Val

We are heading to Bonaire & Aruba in 2 weeks; LOVE GC and heading back in March.

From my research, Bonaire is going to give you more of the rustic feel.

Both islands are fun to grab a jeep and go exploring. However, looks like Bonaire is a more rugged.

On GC, the best snorkeling we have found is between the "North" side and Rum Point -- that is why we stay in that area. The snorkeling here is so awesome that we have found other snorkel areas on GC (and other islands) to not be as "up to par". Fav place to stay cuz of the snorkeling at your front door - Sea Lodges. Check 2 out at http://www.caymanhaven.com/ and
http://www.sealodgical.com/

Have been doing a lot of research on Bonaire - looks like it has a lot of excellent snorkel spots right off the island. However, there is not much beach (GC has beautiful beaches just about all the way around it). A book called The Dive Sites of Aruba, Bonaire, and Curacao by Jack Jackson lists over 175 top dive and snorkel sites - a lot of them are on Bonaire. Will let you know how reliable this book is when I get back.

The book also mentions the currents at the snorkel sites on Bonaire - most range from calm to moderate. Years ago when first started snorkeling, I found the currents at Aruba to be a bit intimidating -- hope to not find this so at Bonaire. I have never noticed currents when snorkeling at GC.

Snorkeling is a high priority for us also. And we've been to St John a number of times, Cayman 3 times, Bonaire once. So maybe my compare and contrast will help. Haven't been to Brazil, but all 3 of the above have better snorkeling than Bermuda and Turks and Caicos. I.e. perhaps the snorkeling one finds taking out a boat in those other places are very good, but in your "new 3" snorkeling right off the beach is better.
You say you like rustic, island, adventure kind of vibe. Cayman has that to the least degree imo. And it's the least attractive island of the 3 imo (by far); it's very flat and has very little lushness. It does have nice beaches though. And fine water. And the snorkeling is good and there are a number of condos to rent that are on beaches that have good snorkeling right there. And I imagine your kids would love the Sting Ray City thing.
Bonaire has good off beach snorkeling, but at most places it's a bit of a scramble to get to it. I don't think little kids could safely handle it. Bonaire is a handsome island and has curious places to drive and explore (flamingo sanctuaries, eg) But Bonaire lacks the beaches that Cayman and St John have in abundance. It is, though, more "rustic" than Cayman and has more of an island flavor.
St John has the best off beach snorkeling of all imo. Plus gorgeous beaches. Plus lots of "rusticity" once you get on to some of the trails. The hiking is wonderful (though may be too exhausting for kids your age). St John is a gorgeous, hilly, very lush island and it's fun to explore by jeep (as are Cayman and Bonaire) and there are so many gorgeous beaches you won't have time to explore them all. If you can swing it, Caneel Bay is the handsomest part of the island we think-- plus they have the best snorkeling. Not a lot of kids there, but there are some. I blieve that families tend to go to the Westin, but there's no good snorkeling there and very little beach.
Safety: none "dangerous," though I wouldn't have the kids wandering around alone
In short, for what you're looking for, St John wins hands down, though I'm not sure exactly what you mean by not "great" but "amazing" snorkeling. I'm not sure I know how to make that distinction. All of it is better than Bermuda and T&C though. If you have further specific questions, I'll be glad to try to help.
Good luck!

I have been to both GC & St John as well. Both have excellent snorkeling from the beach. I'd have to give the edge to St John due to more variety, and some of the most beautiful beaches I've ever seen.

With a 5 & 8 year old, not sure how easy they travel. Grand Cayman is MUCH easier to get to, and depending on where you are coming from, you could be snorkeling by mid afternoon in GC. St John is a much longer travel day, so you may want to consider this as well.

poss's post was spot on comparing the two islands. If you want green, hilly, lush, with close to 60 different beaches, St John is the place....no contest. Just a LONG travel day.

GC will offer a lot more choices of lodging directly on the beach, and IMO, has better restaurants. But the island itself is flat and scrubby. Easy travel day.

I love both places, so I really think for what you are looking for, either choice will make you happy.

sorry typo....St John only has close to 40 beaches, not 60.