[Build Help] R9 290 tri-x vs R9 390 nitro

I'd like to upgrade my pc from an hd7850 to the Sapphire R9 390 nitro or the Sapphire R9 290 Tri-X. But I need your help to choose which one is the best to buy.

First of all I'd like to get some answers from you guys:

• Which one has the best power consumption? (Knowing that I have a power supply of 550w)
• How well each of them will perform with DirectX 12 (or will they perform the same)?
• Is the 4go of Vram of the R9 290 will be enough for the years to come (until the end of the current gen of consoles)?

Which one should I take knowing that I will only play with a 1080p screen, and that in my country the r9 290 costs 285 euro (319 us dollar) and the R9 390 costs 350 euro (392 us dollar) ?