Generally speaking, a TravelPass (also called Qantas Pass) is superior to a Flight Credit as it's more flexible for rebooking. It can be used for an unlimited number of trips, for any people (does not have to be the person whos name is on the TravelPass). Also, there's no rules about the new base fare having to be greater or equal to the cancelled fare. During the pandemic, Qantas automatically converted some users Flight Credits to Travel Passes.
I would go through the exact difference in limitations but I'm pretty sure Qantas has changed their Flight Credit limitations 2 or 3 times, plus the limitations are different depending on whether you cancelled the flight voluntarily or Qantas cancelled it.
There's also differences on how Flight Credits and Travel passes carry DSC promotions differently, and how they're redeemed, and how they can be used to pay for the taxes on a Classic Reward Booking...