As I'm sure you know (since you purchased the FS100) the only difference in the FS series is the amount of built-in-memory or, in the case of the FS100, no built-in-memory. Probably, when you consider the cost of memory for the cameras, the FS100 is the best value of the series. I purchased the FS11 because I'm usually away from any way of downloading the images for long periods of time - backpacking, computer-free vacations, etc.
The quality of the FS series is fantastic, and I've owned a lot of camcorders. I know all the arguments for tape camcorders pertaining to image quality. But, hey, what can I say - I've made the comparison frame-by-frame and the FS series is better than my two Sony tape camcorders. And, the convenience of the flash cards is very important to me. At some point, tape will still be a valid medium, but I'm not convinced there will be enough "old" tape driven camcorders around for everyone to share in order to play the still good media.
Anyway, I wouldn't hesitate at all about keeping the FS100 if you just purchase the amount of flash memory you need. Buy a couple of 16 gig cards - it takes a lot of video to fill up those guys. But, you can always by 3 or 4 or whatever. They're cheap, getting cheaper, and growing in the amount of storage.
I don't work for Canon, have no agenda, and don't care if anyone ever buys one. I'm just trying to be real and tell you that the FS series is a very good option for us folks who want at least DVD quality, but aren't willing to beyond that (HD) at this point due to the storage hassles, transcode hassles, and basically having to settle for SD from an expensive HD camera. HD will arrive, tape will die, flash will be the storage of choice. But, HD hasn't yet arrived and tape camcorders are definitely far fewer in number and dwindling monthly, and flash memory is growing in popularity. The Canon FS series works for me. The memory option (100, 10 or 11 series) is pretty inconsequential if you just purchase the cards you need.
Just one man's opinion, but a flash drive, fit in your pocket, works with iMovie 08, fine quality camera is the way to go - again, for me. Memory is purchasable - I wouldn't take the FS100 back - it's a good value relative to the series (memory is cheap).
Good luck.