The onboard storage is only 8gb, and only accessible when the AC is powered on. 8GB is rather limited, particularly with video.
You can use as big of an SD card you want, as long as its U3 rated. I'd recommend 32 or 64gb. Since it's removable, you don't have to worry about draining an AC battery to access the data. You only need a card reader.
The AC/app will prompt to switch storage if you forget to install an SD card, or later install one. However one can forget they temporarily went to internal storage, forget to switch back to SD, and then wonder where the picture/video files are.