What motherboard do you have? Does it have an M2 slot? It's important to know otherwise you may get a SSD that won't work on your system.
The 970 EVO is much faster then the 860. The 970 is a M2 NVMe SSD that connects to your motherboard through the faster PCIe bus. On the other hand the 860 EVO is using the old SATA connection method which is also used by hard drives. The 970 EVO plus is an upgraded version of the 970 EVO and it offers increased performance over older drive. It was released recently and it was supposed to be at the same price level as the older 970 EVO. However it seems to be more expensive right now, but it's price is expected to fall in the near future. Therefore if you can wait for 2-3 weeks you may be able to get a cheaper 970 EVO plus. Otherwise If you want to get a SSD now, I don't think it's worth paying 100$ more for a 970 EVO plus, since the price difference doesn't reflect the performance difference of both drives. On the other hand if you could somehow find both drives at the same price right now the EVO plus is the obvious choice. Good luck.
EDIT: I forgot to mention that the 86o EVO comes in 2 versions, one is the M2 version and the 2nd one is the 2.5 SATA (Disk) drive version. But both of them are using the SATA protocol and therefore they have exactly the same performance.