According to a TripAdvisor article, it's cheaper to buy at kiosks near Airport Express than to buy at a vending machine?

So I plan to load at least the round-trip fare for Airport Express, which should be about $160? The Octopus card should be $150 which includes $100 of credit.

I know that they will refund the $50 deposit but do they refund the unused credit?

Or will the Octopus Card store that credit for years or is it best not to have credit remaining and to get your deposit back if you may not be back in HK for years?