970 EVO reliability and performance under load has been superb. A 3+ year old controller and flash is likely not going to be as reliable or fast under load.
There is far more than that when it comes to SSDs. Enterprise class SSDs including the P3700 often have power loss protection so you wont lose data if your power goes out, or if you have to power cycle your computer because it became unresponsive. The 970 EVO has a 2GB RAM cache (for the 2TB model), that is a lot of data you can lose. They have orders of magnitude better rates of unrecoverable errors, the P3700 has 10^17 bits per URE, consumer disks are generally 10^14, the 970 EVO doesn't list its URE rate on the datasheet so it probably comparable to other consumer disks. And far better
rated endurance 36.5PB for the P3700, only 1.2PB for the 970 EVO for the 2TB model (even less for smaller ones). There is a good reason why enterprise class disks cost several times their consumer counterparts.
500GB Samsung 970 EVO for $117 x 4 = about $468 and its faster
That may be, but I just picked up 3 HGST SN200 800GB disks for $250 each, and 24 Seagate 600 Pro 480GB disks for about $75 each for a VSAN and S2D/SOFS home lab, for the previously mentioned reasons rather than buying entry level consumer disks even if they may offer better performance. At the end of the day the question is what is more important, data integrity, or performance.