2x390 > 980.The R9 have great scaling. In benchmark they even reach 100%. The main drawbacks are the high power consumption and worse gaming support, as mentioned above. A lot of the AAA games that were released this year didn't have any support for sli stuff at launch. And after a while in a lot of games it was sketchy at best (Far Cry 4,Arkham).
Generally going for the best sinlge card solution is the safest bet. Instead of 2 390 a 980ti would be a better solution IMHO.