G1X Mark III G5X Mark II
24MP 20MP
7FPS AI Servo 20FPS AF Fixed; RAW burst
24MP DPAF / 80D CFA / 1080P 4K / Good rolling shutter / RX100 IV CFA
24mm @ f/4.5 (sharper lens) 24mm @ f/4.9 (better sensor noise, less sharp lens)
72mm @ f/9 120mm @ f/7.6
Weather Sealing Pocket-able
DSLR style; EVF always available RX100 style, pop-up EVF
Phase Detect AF in all AF modes Contrast Detect AF with better hit rate; less modes
Hot-shoe; flash with leaf shutter Bounce flash, no hot shoe
Better SOOC JPEG (Slightly) More RAW Latitude
Swivel Touchscreen Tilt touchscreen
More dials, bigger Less dials, smaller
Better build, heavier Lesser build, lighter
More lens quality More lens reach
More customizability, controls Smarter Auto mode
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Having owned both, I'll say it's a choice depending on your use-case, preferences, and perhaps what type of photographer you are (pro, or amateur/prosumer). Folks like pros are more apt to have a flash gun they may want to use and may prefer a traditional DSLR style body (or not care about pocket ability, will want more customizability).
I think Canon's offering us an alternative (to the G1X Mark III) with the G5X Mark II for folks used to the RX100 style camera. As such, I think it highly likely if Canon does a successor to the G1X Mark III, it'll probably also be a Canon sensor and DSLR-style body.
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Explanations:
PowerShot G1X Mark III's 24MP Sensor has more resolution (obviously) to the 20MP PowerShot G5X II's.
G1X Mark III has phase detect auto focus at 7FPS, but, the G5X Mark II has 20FPS and RAW burst mode.
PowerShot G1X Mark III has Dual Pixel Auto Focus in video, Canon Color Filtering Array (slightly better colors, even though both have Canon's JPEG engine / picture profiles which is half of the "magic", the CFA is the other half of the magic), but only does 1080P. The G5X Mark II has contrast detect autofocus in video (pretty decent with the latest firmware I might add, but still contrast) however it sports 4K.
PowerShot G1X Mark III has faster equivalence on the wide end of the lens, and a sharper one at that (think higher quality optic) but the G5X Mark II has faster equivalence on the long end of the lens as well as more reach, but lesser optical quality (sharpness).
The G1X Mark III you can shoot in harsh conditions, but can't easily pocket vs the G5X Mark II you don't want to bring out in harsh conditions, but, it can pocket easier for those harsh conditions.
DSLR vs RX100 style body is pretty self explanatory.
The G1X Mark III has AI-Servo available to smooth zone, single point and L+Tracking with continuous shooting but in practice, due to the older DIGIC7 processor, it has a lower hit rate in things like Single shot. The G5X Mark II on the other hand can't do continuous shooting at all with AI-Servo, but has DIGIC8 so in single shot it's better. Once again, shooting style differences. Amateurs will want single shot priority, Pros will want Servo priority (in autofocus) in my experience.
The G1X Mark III has a hotshoe and leaf shutter for more advanced flash, but, the G5X Mark II has a novel bounce flash which is quite useful (and more pocket-friendly by skipping the hotshoe mount).
Swivel vs tilt is a preference, some folks like the one or the other, pretty straight forward. Each has it's own perks (tilts are more selfie friendly, swivels are more creative/video friendly). Once again, "pro" vs "amateur".
More dials = more control, but bigger body vs less controls but smaller body
The G1X Mark III has more metal which makes it more durable, and a better less flimsy feel but heavier, the G5X Mark II has more plastic, lighter, but less durable. I can add, the G1X Mark III does have quite a bit more heft in terms of weight, despite how close they are, but it also can take punishment which I can tell from the feel of the G5X Mark II, I wouldn't push it (ruggedness) on the G5X Mark II like I did with my time with the G1X Mark III.
The G1X Mark III definitely eeks out the G5X Mark II in terms of total image quality. However, the G5X Mark II, has more reach. Quality vs Quantity argument here.
And lastly, the G1X Mark III with all it's dials and buttons, and how customizable they are, makes for a more engaging experience, but, the Auto mode on the G5X Mark II is quite a bit better courtesy of newer software and DIGIC8 making it more suitable for folks either more novice, or wanting to focus more on photography itself.
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All to say in my book, I wouldn't say one is better than the other. Rather, Canon has given us an alternative with the G5X Mark II that suits prosumers and amateurs wanting more power, better than the G1X Mark III. But, for pros, or folks that have higher priority on things like landscapes, the G1X Mark III is definitely a good option too. Neither are a bad choice in my book but one may fit your style of shooting better.
