some people would be ok with just a 2.5" sata SSD. for people that are getting this, considering you are on a sata III ssd and you are trying to explain its just much better to have it over a sata I but they just wont budge because they dont think the difference is worth the cost, and they do have a point. sometimes its unnecessary to upgrade but we do it anyway.
right now these pcie cost a lot, and it'll drop overtime, first it'll be 2.5" hopefully we see some tlc at 4TB a piece at 7 or 9.5mm, follow by these nvme ssd become available for majority of notebook.
sata III compare to sata I is around 3 to 3.5x of sequential read/write performance, 4k increases by around 2-3x, and due to older gen SATA I ssd with older flash/controller/firmware etc, latency is about 2-3x higher. NVMe compare to sata III is about the same.