i prefer buying well known gold coin like maple leave gold coin which is highly recognised and more liquid. The bid / ask in BOC is ok (but i only buy to keep and did not sell before. Not sure if there is any hidden cost).
Also, I do not like paper gold as it is not real gold and relies on the creditability of the bank that is a bit contradict to buying gold. Of coz, it is another story if you use gold for speculation but you may consider Gold ETF and check the bid/ask etc which may be more suitable for the ppl who want to trade gold actively.