Please would someone explain the intrinsic difference between Subscriber ID and Subscriber Key in Marketing Cloud for my better understanding.
To add on @Eliot Harper's answer: Careful when you use anything else than the Salesforce ID (i.e. Contact ID, Lead ID, etc.) as your Subscriber key. If you use Salesforce - the choice is clear. If you still don't use Salesforce and you plan a migration to Salesforce - do the migration first, then start using Marketing Cloud.
Changing a Subscriber Key later will involve Salesforce's support at a great cost!