It sounds like either configuration will work fine for you. The mirror with hot spare will provide a lower chance of downtime due to drive failure than the raid 5, but the raid 5 is still a pretty good redundant solution.
With the hot spare, if a single drive fails, you will be back to a redundant state within an hour. With the raid 5, a single drive failure will force you to order another HD, (I doubt you will want to wait for the warranty replacement), so the earliest you will likely be back to a redundant state is the next day, maybe even two or three days later if the drive fails on a Friday.
If you are really confident with your storage space requirements, I would go with the mirrored setup - you won't have to worry about HD failure for a long time. You already have a spare drive you may never need.
If you are running a database without online backup capability, you may want to consider a third option - a mirror set, and then an extra single drive. For backup purposes, you could back your database up disk to disk very quickly to the third disk, than your tape could grab a backup from the spare drive. This will allow for a quicker disk to disk backup than a single raid set will, resulting in less down time. If a drive fails in the mirror set, you can still use the third drive to rebuild the mirror if you choose, (manual intervention will be required), and you will have to reconfigure your backup solution while you wait for a replacement drive.
Good Luck !