I think Chung has much more up side, and not just cause he is young. Chung already has as good of backhand as Nishikori. and Chung's forehand can become better than Nishkori's in the future. and Kei has a great forehand. and Serve, I know Chung will have better one with more consistent power and more accurate than Kei's. We are already starting to see it as we saw his Domination at the New Gen finals last year in Italy.
I'll take a flyer on Chung - mostly because he's 5.5 years younger.
I know Nish's game better, and enjoy his play, but not sure if he'll ever win a "slam". He does have the one US Open final, and made it all the way to World #4 - pretty good career, but hardly out of range for Chung.