U.2, a.k.a. SFF-8639[1], in fact, is the successor of SAS.(It also allows legacy SAS and SATA usage.) So, it has a stronger enterprise inheritance. And so, the enterprise clients can even use the same disk cases/cages without any problem when updating.

Compared to M.2 form, U.2 has two benefits:

1. support hot-swapping(SAS-age enterprise character)

2. better thermal performance/heat dissipation(for larger shape)

for data centers, the future will be the EDSFF family[2](goodness of U.2+goodness of M.2). M.2 can still stay for the consumer market.

[1] https://en.wikipedia.org/wiki/U.2

[2] https://www.anandtech.com/show/13218/ssd-form-factors-prolif...