I've personally only ever used one of the basic boil-and-bite mouthguards, and through years of boxing I've never had any problems.
For a higher-end model, I know people who use both the Shock Doctor and the Brain Pad, but pros get custom fitted for one. I'm not sure where in your area you'd go to do that, but if there's a boxing gym nearby, give them a call and ask.
http://store.titleboxing.com/mouthpieces.html