Alienware X51 R3 m.2 Chip Slot on GPU Riser HELP!

I have a Client who has an Alienware X51 R3. Win 10 Pro x64

Recently I have had to replace the Motherboard in it, we then upgraded the HDD to a 4TB SATA Drive, But Now we are going to put in the M.2 SSD Chip as the Primary Boot Partition.

So I am Looking at the: 

Samsung 950 PRO Series - 512GB PCIe NVMe - M.2 Internal SSD (MZ-V5P512BW)

However my Client is Looking at the:

Samsung XP941 512GB AHCI M.2 80mm PCIe 2.0 x4 SSD - MZHPU512HCGL

Either way I am seeing Mixed Reviews in the Forums about the 512GB being incompatible and needing to have the 256GB for it to be the Primary Boot Partition & Function Properly.

I need to Know what the Best Option will be and if I should buy the PCIe x1 Slot M.2 Adapter rather than use the M.2 Slot on the Back of the GPU Riser..

Also What will be the Best Free Software to Clone the HDD to the SSD I know the Used Space is Way Under the SSD Capacity.