Good point from plugwash. The M.2 SSD system is a confusing mess to most consumers (even somewhat tech savvy ones). The different drive types and keying often results in people buying the wrong drive or enclosure, leaving them wondering why the drive won't fit, or why they aren't getting the advertised performance.
So what does all this mean for the new Raspberry Pi 4B with it's USB 3.0 ports?
SATA-III is rated for 6 Gbps transfer speed, and most SSD models can get pretty close to saturating that. USB 3.0 is rated for 5 Gbps, so a SATA SSD in a USB 3.0 enclosure will be slower than one on an internal SATA-III port. An NVME SSD is many times faster than a SATA SSD, and will also be more expensive, as will an NVMe USB enclosure, but will not be faster on a USB 3.0 port.
This is even more true on a Pi4, which has all 4 USB ports on a single PCIe lane with a shared 4 Gbps throughput (much faster than USB 2.0, but less than full USB 3.0 speed).
So, if you are going to get an SSD and a USB 3.0 enclosure, get the much cheaper SATA parts, because the faster PCIe AHCI or NVMe drives will not offer any perceived performance advantage for the extra money (and it will be a lot of extra money for NVMe).
I actually have a spare 256GB NVMe SSD kicking around, and I've been looking for an enclosure for it, but the cheapest I can find is just under $30, which is the same price as a new 256GB SATA SSD! So it will cost me as much as a new SSD just to put my old one in an enclosure. My GF bought an M.2 SATA SSD enclosure from Amazon for $3 a few months ago (10X less than an NVMe enclosure).
If you don't already have an SSD kicking around that you want to use, I would wait until the USB boot enabled Pi4 firmware is available and some testing has been done. There were some compatibility issues with the Pi3 series and USB booting, which hopefully will be less of a problem with the Pi4, but a wait and see attitude could save you some money.
My mind is like a browser. 27 tabs are open, 9 aren't responding,
lots of pop-ups...and where is that annoying music coming from?