PS4 Pro and XB1 X are the new and improved models. Both are 4K. PS4 Slim and XB1 S are the old and inferior contenders. The XB1 X seems to cost quite a chunk more that the PS4 Pro, though. (US prices: $350 vs $500 on Amazon.)

Quality with these things is relative. The potential provided by the hardware has flipped. The PS4 S beat the XB1 S, but the XB1 X has better specs than the PS4 P. All are AMD inside. The catch will be the OS updates which can change the character of the toy overnight and without warning. (e.g. when Sony removed the Linux option.)

In the end, the determining factor may be the library behind the brands. Sony has more exclusive titles. Have sprog take a look at what else he's interested in that may not be available on the other brand.