USB 3.0 is faster so should give you more potentional for that external drive. But be careful, try to find if it will allow to both read and write on that external HDD. I saw NAS which allowed only read from USB plugged external disk and no write operations.
I would personally won't go with raid 0. If you can rather have it as 2 separate disks and manage it itself it's safer.
For extra security you can use raid 1, but you will loose half of capacity of your disks as one disk is mirrored on another.