Question Upgrade CPU or GPU

[S7hapa]

Hey guys, I currently have avery old rig. I am running a msi gtx 660 ti PE with a i5 3470 and 8 gigs of ram. I have a 144hz monitor. The game I primarily play is fortnite. I am aiming to get 144 frames steady as possible. I play on everything low other than AA set to medium and view distance set to far at 1080p. I get 144 frames but drop a lot during fights. I can either upgrade my gpu or my cpu but not both cuz of budget. I was thinking something like the ryzen 5 2600( i know i need a new mobo and ram). And I know i wont be able to get steady 144 frames if i dont upgrade both but for now which is a better choice to get closer to what I want?

 

[davidgirgis]

GPU