I don't have either, and have only used a Flip Mino HD once when I borrowed it from my sister.
I think that the "complaint" about the Zi8 and Macs is that you can't use the bundled software on a Mac. Since Mac users have access to iMovie, this is a moot point. But someone else who actually owns a Zi8 should chime in here because I'm not 100% certain that what I said above is accurate.
I've been thinking about getting a Zi8 over a Flip because it is more feature-rich: 1080p, mic input, image stabilization. The test videos I've seen on YouTube indicate to me that the image quality of the Kodak is also superior, with much better color and saturation. I've been holding off because I wanted to see what was up with the new Flip slide, but it looks like a step backwards to me.
So, if I were you I'd go with the Kodak.
