Our first dive trip after certification was Key Largo (if you don't count the fact we got certified on Maui!) and it was the perfect trip to "get our feet wet". We're also heading out on our second trip (in 2 years) to Bonaire, and absolutely love the diving freedom there.
You're kind of comparing apples and oranges, especially since the weather and water will be colder in Florida in December than Bonaire, and you don't say what kind of diving you want to do, or if you want any sort of shore activities.
Bonaire is about water sports: diving, snorkeling, windsurfing, and there's no shopping or night life to speak of....very quiet and laid back. The diving is mostly done from the shore, so you drive to a dive site, gear up, and walk in. It requires that you feel comfortable being an independent diver in UW navigation and assessing conditions. Although there is typically little current or poor conditions for entry, it does happen enough for anyone going there to be aware of IMO. There's only one wreck to speak of on Bonaire, the Hilma Hooker, so everything is basically reef diving. Yes, you can do all boat diving on Bonaire....I've read trip reports of people who do just that. Boat dives on Bonaire are guided and the DM may or may not point out critters, and may just lead you around for an hour. Your briefing may be as simple as "follow the DM and limit your dive to an hour".
On the other hand, in the Keys there's a lot to do topside, and if you're looking for night life or shopping, it's hard to beat Key West! All the diving in the Keys is boat diving, and there will be a variety of many big wrecks and reefs to dive. There's also the brand new Vandenberg to dive, which I'm sure is a big draw right now. Because you're boat diving, someone experienced will pick the dive sites based on the conditions that day, pretty much ensuring you'll have good diving. You'll also get a dive briefing about current, navigation, what to look for, as well as the type of profile you should be diving. Most dive ops don't put guides in the water, so you'll want some basic navigation skills, though you can hire a guide if you want.
Dollar for dollar it will probably be a wash since the Keys are easier and cheaper to get to from the US, but island expenses will likely be a wash and dive charters in the Keys are more expensive than Bonaire. If you want to dive, dive, dive then Bonaire is the logical choice. If you want to dive a couple of tanks in the morning and do topside activities in the afternoon/evening, then the Keys would be the best choice.
Just my 2 cents.
