Ext4 will be the easiest probably. The deck supports BTRFS, but requires manual remounting after every steamOS update for some reason.

If you do want to use BTRFS, I’d look into Popsulfr’s deck BTRFS project. I know it can auto configure microSD cards for BTRFS (and fix the previously mentioned mounting issue), as well as set up some of the other BTRFS benefits like automatic compression and deduplication. Deduplication especially can save you a lot of space.