That's going to be a fast moving target over the next few years because the technology is changing so fast. My advice would be to not invest much in whatever is the "best" system right now because it won't be next year. I use a Scala Rider bluetooth headset because an on-line closeout joint had them for awhile for about 60 bucks (they sold out long ago, which explains no link). The other plus is that it's portable and I can move it when I upgrade to a new helmet next year. It's mono, one speaker, but works very well with my RAZR at highway speeds even when I'm wearing earplugs or in-ear speakers for my MP3 player (and I always have one or the other in). I've read that it pairs well a Zumo (I'll let you know after the thaw).
I'll hold off on a larger investment, though, until stereo bluetooth is widely available for both headsets and players. My MP3 player doesn't have bluetooth anyway and I don't think the Zumo bluetooth is stereo, so why bother now?
YMMV, of course.