Go for the experience. Take the train.

There's a 16h overnight train which is not a bad option. The Nanning to Guangzhou tract is already operated by high speed.

The 16 hours train is very comfortable, evening about 6pm from KM and around lunch in GZ.

if time permits, one could take the train to guiyang (6,5 to 8h) and than the high speed train to guangzhou (4-5h), I did not do that, but considered to do it..

@ Hanshan is right about new train, shorter time - 6:30PM to 11Am the next day.

You should also compare prices carefully. The G train (bullet) fares are very expensive. The regular D sleeper trains are much cheaper.
If you have never traveled by the D class sleeper train, there are two classes. Standard class, there are six bunks per compartment, but there is no compartment door. If you are large, it is a bit of a struggle to use the top bunk. The middle bunk is best as you have a bit more headroom. The bottom bunk is communal seating and card table. You are also open to the corridor.
Soft sleeper is four bunks to a compartment, and a door that locks. I prefer the top bunk.

If anyone has traveled by bullet sleeper, please let us know what it is like.

Tiger's info is correct. However, the new train to Guangzhou, Z212 - about 17 hours instead of 24-5 - is the same price as the 24-5 hour train, Y346 for hard sleeper.

Yep, price is the same. Service is better, train look newer,

workers may look younger and more friendly (I take this train just 3 times to GZ and back)

@tigertiger, I've used the high speed sleepers earlier this year. Guangzhou to Shanghai. The standard sleepers were similar to soft-sleepers on the D trains. They also have "deluxe" sleepers but I don't know what the difference is (maybe only two beds/cabin?). The price for my journey was about the same as daytime 2nd class seats. Though onchevking again it was still classed as a D train. Journey time was circa 11-12 hours compared to 7-8 during the day. So a slower train but more modern than other sleepers I've used