Hi Blaine1,
I'm originally from Rio and have dived in the Angra dos Reis/Ilha Grande area many times. In my opinion, you have both very good dive and terribly unsafe dive operators in the region, and unfortunately no way to tell the difference because of a lack of English-language information. I certainly wouldn't book anything until arriving, as everything in Brazil is done last minute and there is no advantage to the consumer. Go into the shop and check the company out for yourself. Ask for certifications, see if they know where there is a hyperbaric chamber (as far as I know, the nearest is in Verolme, next to Angra). These are mostly shallow dives, but you'll get a feel for their competence.
That being said, my experiences match those of the previous commenters: the Gouvea family, owners of Elite Dive Center, run a tight ship. They speak English, have decent equipment that is well-maintained, and keep a steady eye on the weather while you're diving (the ocean side of the island gets rough water). Don't expect safety briefings and diver-down checklists anywhere, though. I've also gone diving with Dive & Cia., which at that time was not affiliated with any international dive organizations as far as I know. My experience with them was frightening and I would not recommend it to anyone. I'm not familiar with Frade or Aquamaster.
Last time I was on the island, in September 2008, I ran into American Lou Servizio, a dive instructor setting up a school (Neptune College of Diving) for instructors. I didn't dive with him, but he might have some great pointers once you're there. My best advice is to find an operator heading for Jorge Grego island, where you have the best chance of running into turtles and the cold-water marine life and corals are quite different from anything you'll see in tropical waters. The Penguin shipwreck on the northwest side is also well worth a dive.
Finally, there are plenty of schools and liveaboards operating on the mainland, but I would avoid spending any time in Angra itself. If you have extra time, try hiking out to the west side of the island, the beaches are some of the most isolated/best in the world.
Hope that helps!