A 380x is fine too. But he asked for the best GPU for that budget. The 970 is a more powerful card then the 380x. And Vram doesn't always help.
And on the other hand 4gb is more than plenty of Vram for 1080/1440p gaming on GTA; And any other current game for that matter.
And most Important...Make sure your PC is powerful enough for handle those cards too.