I'm just not sure what to think of either format, but I do know this is going to get messy. I can see where for enterprise class, this makes perfect sense ... but will be pretty much incompatible with desk boxes except as single-slot options like M.2.
The advantage of reusing the spinning rust shape was that that everyone could use it because everyone had it. Even weird wonky designs could be handled because a SATA cable is a flexible thing.
I love my M.2 for performance, but I couldn't make a proper M.2 RAID5 in my desktop if I wanted to. These new NVMe formats face the same challenge. How much will it matter to manufacturers that they cannot share so much between rackmount and box storage designs? I think that will be the real question, how much it costs them to maintain the differing formats. Because there's just no way in the near future to replace the 2.5" SSD form factor with any of these.