On one hand you have the GTX with it's odd 3.5GB of actual usable VRAM, but still is quite a solid performer. On the other you have a re-badged last gen card with 8GB VRAM.
Sadly, for us, neither of these cards has done much to improve price/performance ratios since the GTX 970 launched. The 970 was a great deal for the equivalent to 780 Ti performance last fall, and the original R9 290 was a great card for the money once non-reference models were available. However, at this point the GTX 970 should have probably dropped in price a bit, and the R9 390 doesn't really do enough to justify its price premium above what R9 290's have been going for over the past year.
That being said, both are good cards.
I personally opted for an EVGA GTX 970 because I will possibly step it up to a GTX 980 Ti through EVGA's step-up program. I wanted to leave myself that option open, although the GTX 970 has been a pleasant surprise and has performed quite well in relation to my past two cards (R9 290 and GTX 780).