I own a Bravo and an Adventure Island, I could make the case for either boat. They certainly both have their strengths. If you like the Bravo, get it. It's a great boat, lots of fun, easy to rig, and perfect to take a guest out on.
For me, I enjoy the Bravo more then the Adventure Island.
So with that in mind, here's why the Adventure Island is better. The Mirage Drive is AWESOME. You can't tip it. (ok, you can, but you won't) You can sail it in strong wind, and light wind, and you'll feel safe. It's also easier to sail to a destination then the Bravo, that's probably because of the Mirage drive, and the ability to not worry about the wind dying. The biggest reason to get the Adventure Island (or Tandem Island) is because the Bravo is more of a workout. In stronger winds, you'll need to hike out, and that will take some core body strength. I find that it was easier to sail my Adventure Island all day then the Bravo. The Mirage drive is also a workout, but using your legs on that can be done all day, and is not mandatory, hiking out on the Bravo is mandatory. That doesn't mean that you can't sail the Bravo for a long long time, but you'll definitely notice your abdominal muscles if you do. This can be avoided if you sail in calmer winds.
So having said that. I still like the Bravo the best, but I can see how the Adventure Island might be an even better beginner boat. It's certainly something different, but if the goal is to get out on the water, and enjoy the process of being wind powered, the Adventure Island is a great option that might get you out there more often then the Bravo.
So yeah... get the Bravo! You'll love it.
Hope that helped! (ok, it probably confused the issue, but feel free to ask any questions that you want to ask.)