Considering that the r9 295x2 is just two 290X's but more expensive, i would rule that one out first.
Now, looking at individual gpus, the GTX 970 is a better performer than the R9 290X. That considered, if the scaling is the same, two 970's would beat out two 290X's.
The 970's will use far less power and power supplies perform better and last longer if they are bit used more than 80% too often.
However, if you're looking to save $80, two 290X's will be the cheaper option and only slightly less performance. But do keep in mind that 290X's are the older hardware, and they are actually going to be getting discontinued once the 300 series comes out this summer, around the time of E3(June). The 970's will be nvidia's top series for at least another year, maybe more.
I am also upgrading to 1440p 144hz in the next month, and if I were buying today I would get getting two 970's. But, I have to wait for my paychecks, so I'm waiting to see what amd releases next month.