I have both a Canonet 28 and a Canonet GIII QL17. I really lucked out and got a pristine 28 off of eBay.
Although I prefer having the full manual control of the QL17, overall I've enjoyed the shots from my 28 much more. The focus ring is definitely easier to use than the QL17's lever. The two major downsides to the 28:
1) Automatic metering. You have no control over the shutter speed/aperature (well, you can manually set the aperature, but then your only shutter speed option is the flash sync 1/30!). Although under normal conditions (i.e. not too dark) the settings the camera chooses have worked well -- it seems to aim for the mid-range and the DOF makes up for any minor focusing errors.
2) Reliance on the meter. If the meter goes or the battery connection goes, the camera is mostly useless (beyond the aforementioned 1/30 default shutter speed). I have one working Canonet 28 and one with a busted meter. Not sure if it's possible (and surely not cost-effective to get someone else) to fix it. At least the QL17 can still work using manual settings.
If you can get your hands on a working Canonet 28, by all means do so. It's a great little camera for throwing into a bag and just taking photos without worrying about changing all the settings).
My Canonet 28 Set on Flickr
My Canonet GIII QL17 Set on Flickr