Hey,
Seeking some advice, though I suspect there’s minimal difference. Between Shanghai- Xian and Xian-Beijing, I’m trying to decide between flying and taking a train.

Shanghai-Xian I’m leaning toward flying and Xian-Beijing toward the train as the total transit time seems to be less doing this way.

Any thoughts?

Yes, I’d recommend what you’re planning.

It depends. It is a lot more complicated to book train tickets and they sold out fast. Much easier to fly.

How is obtaining train tickets complicated ?

But to the OP, your thought are would I'd so as well. Xi'an 西安 to Beijing by train can be done within 4h20. Whereas I know that PKX is a good 40-50mn to the centre of Beijing and PEK also around 35mn so you have add that to your travel time by plane.

Tickets started on sale two weeks in advance so it is difficult to confirm. Some routes are popular and can be sold out in hours. It is OK if you are flexable in your itinerary and don't care about better seats.

Most likely you will need the app to buy tickets. I still cannot register into the 12306 app even though I have a chinese phone number and bank account. End up that I used traval agent to get tickets. YMMV.

That's a little of my thought process.

Shanghai-Xi'an by plane... 45 min to the airport, arrive 2 hours early, 2.5 hour flight time, 45 minutes to city center = 6 hours by plane vs 7 hours + in town transit by train.
Xi'an-Beijing by plane. 45 minutes to the airport, arrive 2 hours early, 2.5 hour flight time, 45 minutes to the hotel = 6 hours by plane vs 4-5 hours + in town transit time by train.

I'm sure one can shave off some time here and there, but the train to Beijing seems like it's a clear winner. To Xi'an, it looks like flying is probably better?

I have Ali Pay set up on my phone and it seems like I am able to book a train ticket... I've gone through everything until entering in my passport info, etc., and it seems to work. According to the AliPay app, it appears that one can reserve a ticket more than 15 days in advance and they will automatically book the tickets once they go on sale. Is this pretty reliable?

You can also use trip.com to book train tickets. But yes, in China, rail tickets go on sale 15 days in advance. That said, unless you're travelling during a peak holiday period, generally there aren't problems especially for big cities like yours. I used to use trip.com on my first trip but have gotten the 12306 app direct and just pick and poke my way through the chinese since I can barely read (I know, shameful being ethnically chinese). I find it quick and easy to use once you go through the setup process.

In mainland China, your ID is your ticket. You scan your ID (passport in your case) to enter the security controls at the train station, and then again to board. As a strong advice : remember aerosols >150ml are prohibited on public transports. Being a guy that shaves, that means I travel with the travel size shaving crčme- learned that the first time. This applies to things like hair spray and cologne too.

I'd fly between Shanghai and Xi'an 西安. Plenty of flights between SHA and Xi'an XIY, and since I am assuming you have status with DL and thus Skyteam, that helps a lot in Asia. There are hourly flights on MU and a lot of them on the C919 so you'll have a chance to try that out . Just counter intuitive to many westerners, the cheapest domestic mainland tickets are often released 2-3 weeks before departure.

Shanghai to SHA is an hour (same as to the train station), unless really off-peak then can be done in 30 min by taxi (PVG is longer). Arrive an hour before - no need for 2h. 3h flight time. 30 min to get off the plane (assuming no bags) + 1h to the destination (assuming no traffic) as the airport is faaaaar from the city. Total: 6.5h+
XA to BJ: 1h to XIY airport by taxi (vs. 25 min to the train station), arrive 1h before the flight, 2h flight, 1-2h on the other end. Train is faster to BJ-West.

We have this conversation every year or so here. When HSR first launched, there was much excitement about it because schedules were predictable and many of us had awful experiences flying. But, now that the PLA has been bounced out of the major flight corridors, flying is much better than it was circa 2010. I used to keep track of my door-to-door times on BJ-SH. As long as you don't go to the airport super early, flying is usually a clear winner. That calculus changes when you look at shorter distances (e.g. Shanghai-Xuzhou which is the midpoint of the Jinghu Line). For Beijing-Xi'an, I prefer flying, not because it's materially faster, but because the experience is more civilized IMO (i.e. no need to deal with crowded train stations, dragging luggage vast distances, or going without real food for 4+ hours).

moondog's assessment is spot on. The only other variable I'd add there is weather - northwest and northeast get snow in winter and most of the country gets rain in July to September (with the South getting rain and typhoons through December), so for me that's also a big factor. Even if you're not flying in those regions your flight might still be impacted. HSR is generally more predictable.

The only two things I liked about living in China, besides the lower airfares, were 1.)The sushi in the midsized city I lived in, Nantong in the Japanese area, $26 for all the sushi you can eat plus all the Asahi draft beer you can drink, and 2.)the high speed trains, especially SHA-Beijing. Unless I could fly from NAN, it took awhile to get anywhere. And not many flights back then.

Personally, I almost always choose high speed rail if it takes less than half a day to get there. It's just a less stressful way to travel.