The essential difference b/n ADR and a common share is that ADR do not have Voting rights. Common share has.
There are some ADR that would in certain conditions get converted to common stock, but by and large most ADR's would remain ADR's without any voting rights.
If you are an individual investor, this difference should not matter as rarely one would hold such a large number of shares to vote on General meeting on various issues.
The other difference is that since many countries have regulations on who can buy common shares, for example in India an Non Resident cannot directly buy any share, hence he would buy ADR. Thus ADR would be priced more in the respective market if there is demand. For example Infosys Technologies, an India Company has ADR on NYSE. This is more expensive around 1.5 times the price of the common share available in India (at current exchange rate).
Thus if you are able to invest with equal ease in HK (have broker / trading account etc), consider the taxation of the gains in HK as well the tax treatment in US for overseas gains then its recommended that you go for Common Stock in HK. Else it would make sense to buy in US.