First, both are SSDs, so I don't know what he is trying to compare. Both use M.2, but different keys from the sounds of it (the "notches").
M.2 drives can be either SATA or NVME, but it is harder to come across a SATA M.2 drive than an NVME drive these days, so I'm having trouble seeing the point.
Though for the Pi a SATA SSD would be fast enough.
The Pi4 compute unit has a PCIe x1 slot, I think, and that could better put an NVME drive to use. But at that cost, for the drive and Pi, I don't see it being widely used outside of hobby or home use.
I'm excited about this and how the official Pi4 firmware now supports USB boot. Maybe if you used the Pi to run a NAS and wanted the NVME drive as accelerated cache (it can do H.265, 4kp60 decode, and H.264)?
But can a Pi even process an NVME drives bandwidth via it's CPU/etc or Gigabit Ethernet/USB 3.0/GPIO add on?
Edit: This guide is basically 1. Put SSD in adapter and 2. Set up USB boot then clone SD card to SSD. How is this article not just a forum post like the dozens of others that provide the same information and process?
There are now lots of low-end M.2 NVMe SSDs on the market, and M.2 SATA SSDs are becoming uncommon. So there's no price remaining price advantage for M.2 SATA over M.2 NVMe—certainly not 40%—and it is not uncommon to find M.2 NVMe sale prices that are better than any M.2 SATA SSD price, since the more popular NVMe SSDs get better and more frequent discounts than the niche M.2 SATA SSDs.
And the M.2 SSDs on the market with the lowest absolute cost (rather than per GB) are usually the Intel Optane Memory 16GB NVMe SSDs that were intended for use as cache devices, but have enough capacity for lots of embedded Linux use cases while also outperforming SATA SSDs of any capacity.
But the keys for the different type of M.2 slots are not large enough on M.2 SATA vs M.2 SSD to prevent them from mixing up. It's easy to force, and forcing it will be fatal for the device.
I know this from an expensive lesson I learned, long ago :(
A M.2 SATA SSD will fit into any socket designed for M.2 SATA or M.2 PCIe SSDs, without damage. A M.2 PCIe SSD that only uses two PCIe lanes will fit into any socket designed for M.2 SATA or M.2 PCIe SSDs, without damage.
The only way to run into trouble is to force a M.2 PCIe SSD keyed for four lanes of PCIe into a M.2 socket keyed for SATA only with no PCIe lanes. Forcing this requires you to install the drive upside down and overcome the 0.5mm misalignment of the 1.2mm wide notch in the drive (meant to be filled by a 1.1mm key in the slot).
If you merely install a SATA SSD in a slot only wired for PCIe, or a PCIe x2 SSD into a slot only wired for SATA, the drive will fail to work but nothing will be damaged.
(You could theoretically also end up in trouble by forcing a SATA or PCIe x2 M.2 SSD into an appropriate slot, but upside down. However, this requires considerably more stupidity on the part of the user because in this scenario the SSD has two notches in the connector, so it is fairly obvious that any mechanical difficulty with installation could be due to having the card upside down. When installing a drive with only one notch, it's less obvious that the difficulty arises from a fundamental and intentional incompatibility.)
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