Not a great idea on the Beijing-Xi'an route to take a D. There is only one of them, a daytime train, and it's only 2-3 hours faster than the overnight Z trains. Not to mention you blow a valuable daytime period traveling on what's not a particularly scenic route, and still have to pay for a hotel room. If not flying, much better to spend this journey asleep in reasonable comfort.

To the OP: If you want a deluxe soft sleeper, you'll need the Z19 train. The other overnight Z train (Z53) usually carries only regular soft sleeper, which is also fine but you'll be sharing with 2 strangers.