970 and 390, both are good performer. But But R9 390 really have a good advantage over 970 that is Vram. AMd offering 8GB vram it is doubled. also Bandwidth is alot higher 512mb comparing to 970. But 390 is latest so you do face the Driver un-optimization issues as always. it will improve afterward.
On the Other HAnd, 970 less power draw............But 3.6 Vram scheme, withdraws the favor, 390 outperform 970 in that case. But Maxwell too good in latest games and smooth as butter in most games.
In case you go with 390, Consider Sapphire TriX is most successful Brand and cooling solution for AMD ever had.