I have both, two GSNs and one Canonet QLIII.
I find that focusing is way faster with the Canonet. The viewfinder is brighter on one of the GSN, then, on 2nd place comes the GIII and my first-bought GSN comes third. All cameras had the viewfinders cleaned.
I have prints made from both cameras, and I must say that the pictures taken with the GSNs are a tad sharper (or maybe a little bit more contrasty), but then again, not by much.
The Canonet has a built quality generations above the GSN, it is smaller but very well made.
I find that I prefer the feeling of the GSN on my hands better, it feels more balanced, just like a real camera should feel (YMMV). That's one of the main reasons why the GSN is my favorite one. Don't get me wrong, I like both cameras, when I go on my picture-taking walks, I usually grab both. They both are amazing picture-taking jewels.
The GSN gets more night time shooting, because its shutter speed can go as low as 30 sec without any photog intervention. If you find something, worth having its picture taken, no matter how dark or dim the light levels are, with the GSN is only a matter of focusing, placing the camera on something steady and shooting away.
With the canonet you are limited to 1/4 of a sec or to use the bulb feature which is not very convenient unless you have a shutter release or the ability to turn into stone for however long the exposure is (providing you guess the exposure right).
One major thing in favor of the Canonet is that you can use the camera in manual mode giving you total control for creative shooting or for when a battery is not available.
On a side note, the last GSN I got from the Evil site, was in inmaculate condition, not a single scratch or dent anywhere on the body. As a matter of fact, the original POD was still on the camera, fully intact. Everything on the camera works, just like the day, it left the factory.
If I were you, I get the Canonet anyway (since you already own a GSN), use it for a while and if down the road you decide that it is not for you, you can sell it again with not loss whatsoever. It's a win-win situation.