ChipFlip
These are the two drives that come up in every "best NVMe" argument, and the used market prices them $150 apart. The WD Black SN850X 2TB asks $400.00, the Samsung 990 Pro 2TB asks $250.00. Both are PCIe 4.0, both are 2TB, both are excellent.
Benchmark differences you will not feel
Sequential read and write numbers on these two are close enough that the winner changes depending on which review you read and which firmware revision was tested. Random performance, which is what actually determines how a system feels, is similarly matched.
For gaming, application launches, and general use, there is no perceptible difference between them. Anyone telling you otherwise is reading a bar chart, not using a computer.
Where they genuinely differ
The 990 Pro has historically run cooler, which matters in a laptop or a motherboard M.2 slot tucked under a GPU. The SN850X pushes slightly harder on sustained writes, which matters for large sequential workloads like video scratch disks.
Both offer heatsink variants. If your board doesn't provide one, buying the version that includes it is worth doing.
The $150 is the whole story
At identical capacity and effectively identical real-world performance, a 60% price premium needs justifying and nothing here justifies it. Used NVMe pricing is noisy because drives are small, easy to ship, and sold by people who have no idea what they're worth.
Check the drive's written-bytes figure before buying either. A used SSD's remaining life is knowable, and a seller who won't share SMART data is telling you something.
Bottom line
Buy the Samsung 990 Pro and put the $150 elsewhere in the build. The SN850X is a great drive, but not a 60%-better one. Ask for SMART health on whichever you buy, because on used storage that number matters more than the model name.