I have multiples of both. I'll try not to be redundant with regard to previous posts unless I have a strong opinion.
* I use both for quick street photography, relying on their metering. When I'm doing my own "eye" metering or using a meter, I use my Canon L1 rangefinder.
* I prefer the Yashica - my main axe is a GTN, but I used to usually use a GSN. The GS/GT is pretty much the same without a hot shoe.
* The Yashica is wonderful for low-light photography because of the insane shutter speed length. Mine have taken exposures far beyond 30 seconds, btw. That's the published spec, but I think it can be much longer. Not sure if it's accurate beyond 30 seconds, but I shoot b/w negatives, so it matters a bit less.
* The Canonet sucks in low light in automatic (shutter priority) mode. The slowest shutter speed is 1/4 second, and if you're using its meter, it just won't let you take a photo if it's not going to be a "correct" exposure. That being said, you can't take a photo with the lens cap on! With the Yashicas... um, yeah, I have some blank frames in my archives.
* The Canon has a wonderful flash in theory - the Canolite D. These are readily available. They adjust intensity based on distance from subject. No need to dial in a guide number or do any math. So indoor photography is a snap. I don't find that it's perfect though, but that could be a defect in the Canonet G-III that I usually use.
* I think the Canonet's lens is a little better wide open. I find the Yashica's breaks up a bit at the edges of the frame once in a great while. But I like the overall contrast and look better with the Yashica.
* The Canonet slips into a jacket pocket, the GTN does not (especially if you use a shade). It's not like carrying around an SLR though. I've never felt fatigued carrying it unless it's an all day excursion, in which case I'm also carrying a bag for film, etc.
I also use an Olympus RC, somewhat dented, and it's a champ. It's super-compact, significantly smaller than the Canonet front-to-back and side-to-side, with a really nice lens (though slower at f:2.8). It's also shutter-priority or manual. I don't even have a strap on mine so I can truly slip it in a pocket. It's often my choice when I'm walking my dogs.
In summary, the Yashica G series and Canonet QL17 G-III are best when used to their advantages--shutter-priority or aperture-priority. I prefer to set aperture when shooting on the street to control DOF, but it's pretty easy to adjust on the Canonet due to the viewfinder info which lets you know the aperture selected. So you can turn the shutter ring and adjust aperture if you'd like.
If I'm going to meter (i.e. if I have the time to meter--I'm not good doing it in my head), I'd rather use a rangefinder without a fixed lens. That's why I use my Canon L1 the most, especially when going to a photogenic destination or photographing a family event. I can carry a bag with my 15mm, 28mm, 50mm and 85mm and do whatever's needed with the best lens I can afford.
If you look in my poorly-maintained gallery, you'll see the Canonet at work on The Gates. Many of the others were taken with the Yashica GSN.
One last note: it's really difficult to find a Canonet everready case in good condition. They did not hold up well overall. It's easy to find a Yashica case in good condition. Though rightfully-dubbed "neverready," they're somewhat essential to block the battery test button from being pushed when you're not using it. I've burned through batteries due to the camera back being pushed up against something, and the case helps prevent that. I keep the bottom on anyway to protect the camera bottom.
Sorry so long - hope it helps.