Head to head
Leica M6 vs Canon Canonet G-III QL17
Both are 35mm rangefinders you focus by hand, both fit in a coat pocket, and both shoot quietly. That is where the overlap stops. The M6 is a modular system body with interchangeable M lenses and nothing but a meter between you and the film. The Canonet is a fixed-lens point-and-shoot-ish rangefinder with a fast 40mm welded to the front and a shutter-priority auto mode. The single biggest difference is exactly that: one is a platform you build around, the other is a finished tool you pick up and use.
How they differ
In the hand the M6 feels like a precision instrument because it is one. The viewfinder is bright with selectable framelines, the rangefinder patch is long and high-contrast, and the cloth focal-plane shutter fires whether or not the battery is alive (you lose only the meter LEDs). You set shutter and aperture yourself and read the arrow display. The Canonet runs a leaf shutter, gives you shutter-priority auto with a match-needle in the finder, and loads film fast thanks to the QL system. Its meter is more central to how it works, and the original cells were built around a 1.35V mercury battery, so most copies need an adapter or a recalibration to meter correctly today.
Rendering is the other split. The M6 takes any M lens, so the look is whatever glass you mount, from a clinical modern Summicron to a swirly old Sonnar adaptation. The Canonet gives you one rendering, the 40mm f/1.7, and it is a genuinely good lens: sharp stopped down, pleasant and a touch soft wide open, useful in low light. Cost and availability are night and day. A working M6 body alone runs into four figures before you buy a single lens, and clean ones are getting scarce. A Canonet is a fraction of that complete, though the good ones increasingly need a CLA for sticky shutters and dead meters.
Choose Leica M6
Pick the M6 if you want a camera you will keep for decades and shoot across many focal lengths, from 28mm street work to a 90mm portrait. It suits the photographer who wants full manual control, a battery-independent shutter, and the best rangefinder focusing experience made. You are buying into the M lens system as much as the body, so it makes sense when you already own or plan to own M glass, and when the long-term reliability and resale are worth the steep entry price.
Full Leica M6 guide →Choose Canon Canonet G-III QL17
Pick the Canonet G-III QL17 if you want one excellent walkaround camera for a small fraction of the money. It is ideal for a first proper rangefinder, for travel where you want a fixed 40mm and fast loading, and for anyone who likes shutter-priority auto for quick street shots. The fast f/1.7 lens handles dim interiors and night streets well. Just budget for a battery adapter and possibly a service, since age affects the meter and shutter more than it does the simpler Leica.
Full Canon Canonet G-III QL17 guide →The verdict
These are not really rivals so much as two answers to different questions. If you want a lifelong system with the finest manual rangefinder feel and the budget to match, the M6 earns it. If you want one sharp fast lens, auto exposure when you want it, and most of your money left over for film, the Canonet is the smarter buy and gives up surprisingly little in daily shooting. Money and intent decide this, not image quality.