Hong Kong Airport Express card also for other forms of transport, of rather a Regular Octupus card?

We will use the AE only once (as arriving by train). We then have 3 days unlimited use of the Octopus card on the MTR, but we would also like to use buses and ferries. For this I understand we can top up our AE card. But what is the best, an AE card with top up or a regular Octopus card (considering we may not use the MTR that much)? Any advise is welcome! And what would be a rough amount that you would need to put on an Octopus card for 5 days there?