Normally I would say software, or rather a raid-like filesystem like btrfs or ZFS. But in your specific case of funneleing it all through a single usb-c connection it is probably better to keep using an external box that handles it all internally.
That said, the CPU load of software raid it very small, so that isn't really something to be concerned with, but usb connections are quite unstable and not good for directly connecting drives in a raid.