I think you are talking about those cabin with only 2 beds, are you?

They are alright, and enough privacy for two of you and very safe. You won't be able to see anything becasue of the dark outside, except seeing the sun raise will be nice on the train if the weather is good.

Train stations are all in city centres, and airports (especially PEK) are a bit further away. So you need to calculate taxi cost as well, it seems you are quite price sensitive.

For me, I would go for the flight, it save me time (I only do business trips in China, not as a tourist, so time is important to me), and one night in the hotel will get me extra hotel points

If you are not converned of both, and want to enjoy a different experience, train is an excellent option on this route.