Why go
Banks, independent exchange counters and cash withdrawals offer different ways to obtain Hong Kong dollars. The useful comparison is the final amount available to spend. This matters especially for smaller exchanges, where a fixed charge can outweigh an apparently better rate, and for withdrawals subject to your own bank's limits.
The full story
The Hong Kong dollar is identified by HKD or HK$. A plain dollar sign in local pricing refers to that currency unless another designation, such as US$, is shown. Checking the currency matters when comparing a Hong Kong purchase with a price from home. For cash exchange, ask how many Hong Kong dollars you will receive for the sum you hand over, including any commission. A fixed fee has a larger effect on a small transaction than on a large one. Comparing that final figure across counters is more useful than comparing a prominent rate in isolation. An ATM offers another route to cash, but an overseas card's network, withdrawal limit and charges still matter. Check those with your bank before relying on withdrawals during the trip. Keep the choice of where to obtain money separate from the question of how a particular shop accepts payment; solving one does not automatically solve the other.