An SSD form factor is the size, configuration or physical arrangement of a SSD media.The form factor normally determines the media’s physical size and interchangeability with other computer components or devices. 

M.2 SSD

M.2 has become the standard type of storage for slim laptops and notebooks.Its tiny form factor is often compared to a stick of gum and easy to install right on the motherboard in most cases. It is available in various lengths to enable different SSD capacities, The longer the drive,the more Nand flash chips can be mounted on it leading to higher capacity SSDs.

NVMe SSD

NVMe (Non-Volatile Memory Express) is a communications interface and driver that takes advantage of the increased bandwidth PCIe has to offer. It’s designed to increase performance and efficiency while making a broad range of enterprise and client systems interoperable. NVMe was designed for SSDs and communicates between the storage interface and the system’s CPU using high-speed PCIe sockets without the limitations of form factor.

While M.2 and NVMe are popularly used now,they actually refer to different technologies that can be used together but:

• NVMe is a storage transfer protocol specially designed for high-performance,non-volatile storage media.
• M.2 is an SSD form factor that enables high-performance storage. 
• M.2 SSD connected using the NVMe interface deliver faster read/write speed

There are two types of M.2 SSDs which are SATA and NVMe based.They differ in storage technology and depending on your needs and budget,they each have their own pros and cons. 
M.2 SATA SSDs use the SATA interface with a max. data transfer of 6Gbps. This is slow compared to the new NVMe intefaces. But it have 3 to 4 times the bandwidth compared to traditional HDD and is more available and affordable than a NVMe SSD. So it is a great alternative

 M.2 NVMe SSDs  are much more performance driven compared to M.2 SATA SSDs. By leveraging the PCIe bus, M.2 NVMe SSDs have theoretical transfer speeds of up to 20Gbps which is already faster compared to M.2 SATA SSDs with 6Gbps. PCIe buses can support 1x, 4x, 8x, and 16x lanes.

Also,it can deliver stable performance even in extreme temperatures ranging from -40°C to 85°C. In actual applications, It is ideal storage device for rugged tablet PC which needs superior reliability and durability besides high speed for storage product in hash conditions.