Hello Richard,
[Topic moved here.]
Thanks very much for the intention to buy a licence.
Functionally, the only differences are that, in the case of a subscription licence, you need to keep paying the subscription, and refresh the Hauptwerk licence in iLok License Manager once per subscription period (e.g. year, for yearly subscriptions) in order to be able to keep using Hauptwerk (otherwise Hauptwerk would stop working unless/until you subsequently restarted the subscription or purchased a perpetual licence). A perpetual licence will never stop working (assuming you continue to have a computer that's compatible with the platform prerequisites of the Hauptwerk version that you had purchased).
Subscription licences include upgrades to subsequent major Hauptwerk versions, whereas with perpetual licences you would need to pay to upgrade to subsequent major version numbers. (The upgrade from a v5 perpetual Advanced Edition licence to a v6 perpetual Advanced Edition licence was $99, for example.)