First class can vary on that route a bit by train. You'll likely get something close to domestic first class airline seats. The difference is 5 across versus 4 across (first vs second class), both with ample leg room. I've done both several times, but I definitely prefer by plane as I use my club membership for food and drink (diners). United miles can be used, but not likely a good value. However, there are a few trains running on that route with international business class airplane style seats for first class at the moment, not sure if its permanent. As all things China, completely hazy. The other thing I dislike about Chinese Highspeed rail is no WiFi and using phone data is out of the question going at highspeed. So it's a long time just staring out a window at blurry objects on that route.

That being said, July and June are not the highest periods of travel but not the lowest either. So you probably don't need to book too far in advance. 9AIR a budget airline is to start service on the gz/beijing route early next year which will probably bring down the prices quite significantly.