RAID 1 is secure, if a drive fails, you have a spare.
No much performance increase in reads, but writes take a small hit.
You do not need to run it 24/7, unless you require that access.
I am installing 4 SATA drives, two for RAID0 boot drive, two for RAID1 data files.
I'd go with 3 hard drives and run a Raid-5.. it's fast in reading/writing and has the highest fault tolerance.
If you use a hardware setup, you should be able to span your OS across the 3 drives. But you might want to just pull off 2GB and put your OS there, Raid 5 the remaining. All together you'll lose 6GB plus the size of 1 HD for parity.
But if you run 2 disks, do the mirroring. It's simple to setup and if one drives fails, just break the mirror, pop your backup in, and rebuild the mirror with a new hard drive when you can.
But you definately want to run a hardware raid.
The hard drives will always spin.. even when not in use.. so with a mirror, if you write something to your drive, it'll write it to the 2nd drive also. It won't be a problem for what you're trying to do and I would definately recommend either Raid 1 or Raid 5.