For the benefit of other readers who may be inclined to take the train, note that the overnight D train from Guangzhou to Beijing does not run every day just around the weekend.
One's choice is all a matter of personal taste. I find second class perfectly fine, just about the same space as I'va had on many business class flights) and that the price difference for 1st is not worth it. But that's just me and I am paying for my tickets myself.