The Zi8 has very poor audio input compared to the Q3HD. It uses fairly aggressive auto-gain control and rolls off the low frequencies heavily. Even with an external mic, I prefer the sound from the Q.
The Q3HD needs plenty of light to do a good job, but that is true of all cameras. Better low light performance costs money, that's about all there is to it. Of course, each of us has a different threshold of tolerance <grin>.
The video quality from the current generation of Panasonic point & shoot cameras is very very good, but you still need plenty of light. The audio can be decent if the the dynamic range is not too great, based on my experience with the ZS3 and TS2, but quiet passages will get noisy. I can generally get 1 hour from my ZS3 battery. AC adapters are available for most small cams including the Panasonics, the Zi8, and the Q3HD, but not the Flip.
Your Naiant combined with the Q3HD will be pretty hard to beat at a sub-$1000 price point, the quality of the video will be a function of the amount of available light.
I have some comparisons of the Q3HD with the Zi8 and a Sanyo video camera at my Homebrewed Music site:
http://www.homebrewedmusic.com/2010/11/28/a-first-look-at-the-zoom-q3hd/Also a comparison to the iPhone 4:
http://www.homebrewedmusic.com/2010/12/01/zoom-q3hd-vs-iphone-4/Using the Zoom H2 as a powered mic for the Q:
http://www.homebrewedmusic.com/2010/12/01/zoom-q3hd-with-and-without-the-zoom-h2/And some different mics plugged into the Zi8:
http://www.homebrewedmusic.com/2010/03/18/some-mics-for-the-kodak-zi8/I just put up a video shot mostly on my Lumix GH2 (this is what you _really_ want for video quality in marginal light, bring money for a fast lens)
http://www.youtube.com/watch?v=RwyDhCZWIwgThe audio in this clip is from the Q3HD, placed about 24" from the guitar. At about 1:20 into the clip I used the video from the Zoom for one verse. With plenty of light, it does a wonderful job to my eye, and I'm quite pleased with the audio throughout.
Fran