I don’t have any personal experience with either one of those. But I think I would go for the S4. The fact that Xone DX is discontinued and second hand would be the reason for that. If you buy your controller new you have warranty …. and is new. I mean, do you know how heavy the Xone DX was used? That is also something to consider. Is it used in a bedroom or in club? Did it travel a lot from one gig to another? How many different people have used it?
So my advice would be to buy a new controller if your budget allows it. But like I said that is just based on the Xone DX being second hand.