I have no idea why the 950 Pro is more expensive. The 960 Pro is newer, and probably came out with a lower MSRP (due to more competition in the market)
Samsung don't sell them themselves, and direct you to vendors. The vendors probably paid more for the 950 pro (due to the higher MSRP) and therefore have to recoup their outlay + profit. The 960 pro was cheaper for them to acquire = lower selling price.
Either will work on your board, yes.
960 Pro is a no-brainer.