I dont know why it would be but the saphire is the cheapest ( from my checking on amazon.com) so i would just get the saphire one because its cheaper by about $60 and since there is no custom coolers they will be neck and neck
Don't buy the 7990, in less than a month AMD will have a new line of cards out, they have to have something to beat the 780, and if it beats the 780, it will be faster than the 7990, the difference being its a single GPU.