Hi, I currently have a 750ti, and I'm looking to upgrade to either a gtx 970 or Sapphire Radeon R9 Tri-X 390X. My question is

Solution
As long as your PSU has 6+2pin and 6pin then you should go for the r9 390x, it's the better choice. However either list full PSU model so I can give you that info, or make sure for yourself. But the 390x is the more powerful choice.