halfpny

I'll be traveling to China in early March (arriving on the 10th) for 8 days. I'm flying into and out of Beijing, and trying to make the most of my time. The only cities I'm trying to visit are Beijing and Xi'an, which seems like a reasonable amount of sightseeing in my limited time. I think the idea of a sleeper train seems exciting - the difference in price between flight/hotel and sleeper train is negligible, but I also want to make sure I'm making the best use of my time.

With that being said, is taking a sleeper train to Xi'an and then flying back the following day (or 2 days later) the best use of my time? In addition, I noted on several other threads (from the fall) that sleeper trains tend to sell out quickly - is this true in the Spring as well? Does anyone here have any better solutions to booking these tickets in advance than to try calling my hotel/hostel and seeing if they can book them for me? Are there any reasonable websites that I'm missing or are they all going to try to gouge me? Will I more than likely miss out on the train opportunity if I wait until a few days before my departure?

Thanks for any advice you can offer!