Hello fellow bros who are enquiring whether to up your GPU or CPU. I would all the other advices given are good. Just stick with GPU first. Monitor your CPU usage then decide. CPU upgrade is a major system upgrade with many parts needed to change to suit the new generation.
For myself, I am still using my i5-2500. How far have my system gone through? Well from GTX 560, R9 270x, R9 390x and finally a GTX 1080. My gaming experience has improved by alot. My CPU is still going strong although my usb ports on my MB have died. I have budget constraint on my end so overall I really still enjoy my gaming experience. I am able to enjoy Overwatch, Titanfall, Battlefield 3 to 1, all the COD titles with my 1080p monitor happily. So I would say GPU is a better choice. Besides u can bring your GPU over in an event of system change.
Decide which card is better value for youself liao, its own preference for this end.