Short answer. Octopus is a must for its convenience. Each bus line, each MTR ride, each ferry route cost different in Hong Kong. You'll be carrying huge amount of coins on your without an Octopus.

Unless you only take taxis...

Make sure you load up your Octopus before going to the islands as ferry to the are relatively expensive, and so are the buses on Lantau. [Tip: take the "slow" ferries over the "fast" ones when available. Half the cost, better views, and only take 15-20 minutes extra.] Few places to load up the card on the islands as well.

Macau has its own stored-value card, but it's really not necessary to get one, unless you're staying there for a week. Taxis are plentiful and cheap. And you can walk to a lot of places.