I had exactly the same question when I got my A3 last month.
The Hill Assist simply holds the vehicle for a few seconds after releasing the footbrake so you get no roll-back.
With the electronic parking brake (unlike a mechanical one) you can simply drive away and when the power reaches the right point, the brake magically releases. It's actually very good! If you do decide to release it manually, you must have your foot on the brake first.