The 7990 is a old dual GPU card that should perform slightly better than the 970, but it's going to be hot and noisy and annoying. The GTX 970 will save a lot of crossfire trouble.
8% performance difference at 1080p, up to 25% at 5760x1080p.
Its up to you if that 8% is worth the heat and noise from the 7990. There is also the case where games do not support Xfire, in which case you have a single 7970.
If you're gaming at FHD, the 970 will serve all your needs and be a lot less annoying. If you plan to multi monitor or go 4K, then the 7990 is much better. What's the rest of your system?