Picking what's faster depends on how you are going to use the card .... reference or non-reference ? ... out of the box or overclocked bawlz to the wall ?
Further thoughts from TPUs reviews .... did I get any of the math wrong ?
$435 390x is 2% faster than $320 970 / 7% Slower than $510 980 at 1080p
$435 390x is 7% faster than $320 970 / 4% Slower than $510 980 at 1440p
From tech powerup
MSI 390x is 2% faster than reference
MSI 970 is 3% faster than reference
MSI 980 is 8% faster than reference
Performance of MSI 390x increased 5.1% manually overclocked
Performance of MSI 970 increased 14.0% manually overclocked
Performance of MSI 980 increased 14.6% manually overclocked
At 1440p ... In reference to the 970...the 390x is (98/91) faster .... looking at MSI's versions of both that becomes (98/91) x (1.02/1.03) .... looking at the manual overclocking ability that becomes (98/91) x (1.02/1.03) x 1.051/1.140) = manually overclocked MSI 390x being 98% as fast the MSI 970.
For the 980 .... (102/98) x (1.08/1.02) x 1.146/1.051) ... it's 20% when both using manually overclocked non reference MSI cards.
The 390x brings some welcome competition but if you are using non reference cards w/ afterburner .... I don't in any way see paying $100 for the 390x over the 970. OTOH, Paying $80* more for 20% more speed with 980 as opposed to the 390x is a much harder decision.