ChipFlip
Power supplies are where builders economise and then regret it. These two make the case for not doing that: the Corsair RM1000e at $114.99 used and the NZXT C850 at $105.00. Ten dollars apart, 150 watts apart, and both ATX 3.1.
The 150W matters more than it used to
Modern GPUs draw large transient spikes well above their rated power. A 5090 or a 4090 can momentarily pull far more than its nameplate figure, and a supply sized with no headroom will trip its protection and shut the machine down under exactly the loads you bought the GPU for.
That's what the extra 150W buys: margin for spikes rather than sustained draw.
Both are ATX 3.1, which is the specification that matters
ATX 3.1 supplies are designed for those transients and ship with the 12V-2x6 connector current high-end cards use. Buying an older supply and adapting the connector works, but it's the arrangement that produced a run of melted connectors, and it's avoidable at this price.
Neither of these has that problem. That's the main reason to prefer either over a cheaper older unit.
Where the NZXT makes sense
An 850W supply is genuinely enough for most builds, including a single mid-to-high-end GPU. If your card is a 5070 or below and you aren't planning to upgrade upward, you're paying $10 less for capacity you'd never use.
Both are fully modular, both are Gold rated, both come from companies that honour warranties.
Bottom line
Take the RM1000e. At a $10 gap, 150W of headroom is the cheapest insurance in the build, and it leaves room for a future GPU upgrade that the 850W unit wouldn't survive. Take the C850 only if your total system draw is comfortably low and you know it's staying that way.