I would also get a 390 over a 970. They are bigger, use more power and in general just far less nooby. BUT if you can use more than 3.5GB vram with a single card and still get good fps... then I guess your playing modded skyrim.
Lets talk GTA V and 1440p, probably two of my favorite things! Unless you have two 6GB cards you won't hit absolute maximum settings. But it will still be beautiful. 3GB is definitely enough, definitely definitely for one card. And one card is with certainty enough to run the game well. In my main rig (the only one I play GTA on) I run 780's. For the first two days playing the game I didn't actually notice I had SLI disabled.

So straight off I think you'll have a great time with a 970/390 because they are nearly as good as 780's, sadly they are hard to get hold of now. (don't tell me Kepler isn't the finest generation of GPU's in the world). Now I have 3 cards I have maxed out just about everything besides AA which unfortunately vram won't allow and it runs at around 110fps. With two cards it's probably the perfect balance between performance and vram, staying consistently above 50fps with everything enabled. So 3.5GB on one card is definitely enough for 1440p GTA V and still enough for two cards IMO. 8GB would start to be useful with 3 cards.
Are you contemplating a new monitor or new card. Definitely go 1440p if you haven't already, 25" is legit too.
JR