ChipFlip
$1.24 apart, 150 watts apart, and one is fully modular while the other isn't. The EVGA 1000 GQ runs $71.99 used against the Corsair RM850 at $70.75, which turns out to be a cleaner comparison than the wattage suggests.
Semi-modular versus fully modular is a build-quality issue
The GQ is semi-modular: the 24-pin and CPU cables are permanently attached, and everything else detaches. The RM850 is fully modular, so you only install what you use.
In a case with limited cable room that difference is real. Permanently attached cables you don't need still have to go somewhere, and "somewhere" is usually crammed behind the motherboard tray fighting the side panel.
The wattage advantage is smaller than it looks
1000W versus 850W sounds decisive, but both comfortably run a single-GPU build with any current card short of a 5090. Unless you're at the very top of the power curve, the extra 150W on the GQ is capacity you're carrying rather than using.
Meanwhile the RM850 is the newer design of the two and the RM line has held up well.
Age is the thing to ask about
The GQ is an older EVGA line, and with EVGA's reduced presence the warranty question applies here the same way it does across their range. A used semi-modular supply from a company that's stepped back from the market is a different risk profile than a Corsair unit of similar age.
Both are Gold rated, so efficiency isn't a differentiator.
Bottom line
Take the RM850. Fully modular, newer design, better support outlook, and $1.24 cheaper. The GQ's extra 150W only earns its place if you're specifically planning a build that needs it, and if you are, you should be looking at the 1000W-plus options with ATX 3.1 support instead.