A Closer Look
Originally I wasn't going to remove the stickers on the SM951 and SM951-NVMe, since both parts should be identical. But as I took pictures, I found that they're aren't exactly the same, though.
In the shot above, the NVMe model is on the top; you can identify it by the third letter in the part number. The NVMe model is designated by a V, while the AHCI model has an H.
The same Samsung three-core UBX controller is used on both SSDs.
The same low-power DDR2 DRAM is also used on both 256GB parts.
We took a picture of the back of the drives to show that this capacity size is single-sided. But in doing so, we noticed that one drive was a little taller. After investigating further, we discovered different flash. The SM951 AHCI uses an eight-die stack (per Samsung's NAND flash decoder) and the SM951-NVMe uses a 16-die stack.
For the most part, though, the flash is the same other than its die density and the number of dies in each package.