Never heard of the Corsair One. Looks like a nice machine. How is the fan noise and is there any throttling?
I'm thinking of getting the F131 from Maingear or Bolt from Digital Storm. Small tower with full acrylic side panel and hard-line tubing...what a dream.
But I'm open to a mini tower that looks sleek & minimalist like that Corsair.
IMO, if one is at all serious with gaming, just get a dedicated PC for it (and a ultrawide monitor).
Mac for anything else.
The Corsair One is actually very good. I am very impressed with it. Not only it is beautifully designed in a small-form factor, it is also very quite and cool. It is the quietest PC I have seen. While playing World of Warcraft at all max setting on an Ultra-Wide 34 inch Alienware at 1440P and 120Hz, and WOW is getting 120 FPS constantly, the Corsair One is still surprisingly very quite. The CPU temp hover around 50-60C. My iMac 5K 2014 version on the other hand, you can hear the fan spinning loudly with a setting of 5/10, and after sometime it will just boot me out of WOW.
To answer your specific question:
If you are just browsing internet, using Words etc. The iMac is probably quieter. You can still here an extremely faint fan sound on the Corsair with very light work, assuming our house or room is extremely quiet like mine.
On the other hand, under heavy load, Corsair One will be significant more quiet than then iMac.
The only problem is, it is a PC. It has so more wires compare to the iMac. I like minimalism and clean.
From a gaming perspective, it may be the perfect machine. I have the 1080TI, 7700K, it is very speedy doing anything. Customer support is also great.