Train or Ferry? Which one's cheaper and more convienent?

Train. I don't think there's even direct ferry service from Kowloon to Guangzhou. Only from the airport, I believe.

Cheaper will be by bus, and it has more departure points, but require getting off the bus to go through immigration/customs at both the Hong Kong and China side.

From central Train station in HK to central train station in Guangzhou? how long of a train is it? How much?

~$190, ~1hr45 minutes.

http://www.kcrc.com.hk/eng/services/...itts_intro.asp

Train departs from Hung Hom station in Hong Kong, where you go through HK immigration for leaving the territory. Chinese immigration at Guangzhou East station.

Guangzhou East is not the main station, but it is served by the subway train there; or just take a taxi to your destination.

That's ~HK$190, each way.

rkkwan- so helpful. Thanks!