I vote for cans because there a variety of brands and styles in my area, but that doesn't mean I wouldn't participate in a different type of round.
If you're going to do away with the preferences, I would probably make it style based. It would be a bummer to send a beer someone didn't enjoy, even though I'm sure the recipient would be gracious. Perhaps you can allow people to exclude one style? So a person could say, send me anything but hefeweizen (or whatever style they don't like). Only one thing to keep track of and everything else is fair game.
If you're going to do away with the preferences, I would probably make it style based. It would be a bummer to send a beer someone didn't enjoy, even though I'm sure the recipient would be gracious. Perhaps you can allow people to exclude one style? So a person could say, send me anything but hefeweizen (or whatever style they don't like). Only one thing to keep track of and everything else is fair game.