Take a look at this teardown of a 2.5" Samsung 860 Pro: https://www.myfixguide.com/samsung-860-pro-ssd-teardown/ https://www.myfixguide.com/samsung-860-pro-ssd

Take a look at this teardown of a 2.5" Samsung 860 Pro: https://www.myfixguide.com/samsung-860-pro-ssd-teardown/ https://www.myfixguide.com/samsung-860-pro-ssd-teardown/ Notice how little of the case is actually occupied by circuitry: https://i.imgur.com/lzPTC3T.jpg https://i.imgur.com/lzPTC3T.jpg I think the better question than M.2 vs. U.2 is the "plugs directly into motherboard" form factor vs "uses a cable from the motherboard to plug into an enclosure for a separate circuit board that needs separate mounting points in the case" form factor. Since forever, hard drives have come in 3.5" and 2.5" form factors, so when SSDs hit the market it made sense for them to come in backwards-compatible enclosures, in the same way it made sense for them to implement backwards-compatible protocols like SATA. NVMe, as a protocol, breaks backwards compatibility, to great benefit, so I don't think it's surprising that breaking backwards compatibility on form factor happens at the same time. U.2 seems ideal if you've already got a server chassis design that accepts 2.5" hot-swap SSDs, and you want to use the NVMe command set instead of SATA. For most consumer purposes, having M.2 storage that you just plug into the motherboard like you do RAM and the CPU is completely worth the switch away from the 2.5" drives.