My experience 2.20PM coach from HK airport to GZ, we reached GZ at 8.40pm in the evening and no toilet stops in between!
Express train from Guangzhou Dong Zhan (GZ east train station) to HK HungHom station. 1hr 30mins. Express train cost RMB 253 (premium class)
Coach is not worth it at all! And for those wondering, you need to carry all your luggage across the immigration check point if taking coach. Not for the train riders tho'.

