GTX 980 and R9 390X are not pricepoint fighting, that would be GTX 980 vs R9 Fury.
The Fury beats the 980 in most cases but the 980 is better at overclocking.
It’s really just what you prefer and what you get a good price for.
Ween it comes to 390 vs 390X I personaly would and will go for a 390 unless the 390X is on sale.
Also get yourself an SSD if you don’t have one.
Edit: if you go 1440p and spend about 600 bucks go for the 390X so you can get the most out of it.
I would strongly advice this monitor for 1440p AMD gaming: