The 950 PRO's advantages are:
1) it is a "retail" unit so you have warranty support from samsung direct. The 951 is "OEM" unit, and your warranty is only from the vendor. The 950 PRO's warranty is 5 yrs vs the 3 yrs for the SM951 i've seen the vendors quoting.
2) as a retail unit, samsung provides the necessary driver for NVMe use with win 7, 8.1 & 10
3) the 950 has newer tech, ie V-Nand
Personally, i'd go with the 950, but either one, read the reviews and note you should seriously consider installing active cooling, ie a fan moving air across the controller of either SSD