the only difference really between the the 290x and 390x/390 is that one has full on support for DX12 and has massive clock-speeds. the other doesn't. (290x being the one that doesn't of course).
If you're willing to spend 350 dollars on a 290x 8GiB card. i would really recommend you push your attention towards the 390. it's new and has support for DX12.
they're both good. the only difference between them is the MSI is extremely overclocked out the box compared to the Sapphire and every other 390 model. 1100mhz of horsepower. and 1500mhz on the memory. (or 6000mhz if you want to get technical.)
A any good 390 will outpace a 290x, and considering that the 390 is a beast at over clocking and at the same price, while the saphire 290x 8gb is fantastic, the 390 is just ~5%-10% faster depending on clock speeds