We took the bus from Guilin to Yangshuo 2 weeks ago. I can very much recommend to take the one from the bus station and NOT from the train station. The price is just 15 instead of 14 (if I remember right) but the bus goes non-stop and everything is well organized.
Apparently you have to bargain a lot if you take the bus from the train station, you probably end up paying more, the bus is stopping all the time, has less space for luggage and you have to be careful not to be victim of theft.
I had found a link to an external website on Tripadvisor which had explained all this. I can't find it anymore, but it was very useful. I want to make sure other people who want to make the same trip know this.