They do not have the same performance, and the 970 EVO Plus wasn't released until 8 months after the original 970 EVO.
The 970 EVO Plus has faster sequential read and write speeds AND has faster random read and write speeds, pretty much maxing out the PCIe 3.0 bus when it comes to theoretical speeds, which the original 970 EVO did not do.
Given the choice between the two, the Evo Plus is a no brainer so long as the price difference isn't very significant but either drive is an excellent choice that doesn't tend to have as much potential for thermal throttling as many other NVME models on the market.