I generally like trains, but HSR is pretty characterless and not much fun, plus there's nothing much to see out the window on the Guangzhou-Beijing routing. As well, at least on some HSR trains there's a constant high-pitched whirring sound that might get to you on a very long trip.

So I'd be inclined to fly. Unless I wanted a more traditional Chinese train trip just for the experience, in which case I'd take the regular overnight sleeper.