Having been to both in the past year I'd definitely say that Cayman was better. It depends what you like though.
GC is much busier than Bonaire and imho the diving is better, although it's mostly boatdiving. Bonaire is much more laid back and is great for shore diving, although all the west side diving kind of looks the same, it's all ironshore or beach with coral rubble entry with a drop to about 120' down a coral infested wall. If you do decide on Bonaire, book some boat dives to Klein, the water is clearer and the fish life is better. The live corals on Bonaire are incredible but the fishlife isn't nearly as good as on Cayman's north/west sides.
GC has much more dive variety, the North Wall, the deeper West side dives, Stingray City, scootering on the North wall etc., although the shore diving is not as prolific. There is good shore diving at Sunset House and both Divetech sites to the North.
On Bonaire we had a condo, but we went by the Divi everyday and it looked alright. It was one of the hotels we had considered, if I was looking for a full-feature resort I would have stayed there. Although I think it's also a little older, part of that impression is because it looks like a jumble of buildings that were all added over time - which it was. And it's walking distance to Kralendijk.
ON GC we stayed at the TICondo's, right next to the Treasure Island resort - personally I think you're better off at the Comfort Suites or Indies Suites or the Marriot.
TI is kind of old and tired looking - it was definitely built several decades ago. We had considered staying there but were told by our travel agent that the resort at one time was used a lot by large package tours in the past and was pretty beat up over time. Having seen it I'd have to say I agree with that opinion. The pools and other facilities were nice, but the rooms looked very dated - I looked in one that was being cleaned. And the beach is non-existent, it's just ironshore with a covering of sand. There's much nicer beaches farther up SMB, even the beach at the condo's next door is much nicer. And the beach on Bonaire was - Oh wait, there wasn't a beach on Bonaire! - unless you call the little man-made strip in front of some of the resorts a beach.
One good thing about staying at TI resort is you can walk to TIDivers pier if you're diving with them. Although all the other dive ops will pick you up right in front of TI's main lobby. And Soto's has a diveshop in the main lobby area but they don't dive from there, they'll take you to their boat on their van.
Google Treasure Island resort, there's a link somewhere about problems people have had at Treasure Island ranging from staff discourtesy to problems with plumbing in the rooms.
We found some great restaurants on both places, although GC was much more expensive. And I thought food was very pricey on Bonaire last month until I remembered how ultra-expensive it was on Grand Cayman.
Also rent a car at either place, it's a must on Bonaire for shore diving and it makes it easier getting around Cayman as well, if you do stay at TI, you'll be driving to every restaurant except the Greek place next door.
Steve
After the 3rd - 4th week of October passes the odds are pretty good. But I do appreciate the warning.
