Data only changes every couple of months ? Do you not use your computer for anything except installing programs onto (and then not use them) ?
Here's the deal. I would say, personally, go with RAID 1. That will provide you with instant failover protection, instead of rebooting, repairing, etc.
Then you have to ask : what happens if your backup drive itself fails ? RAID 1 protects against either drive failing, practically at any time. So, all-around, RAID will give you better backup protection.
However, if you don't have a RAID controller and need to go software, and don't have an HD controller that's capable of multiple concurrent write operations, you will slow down performance somewhat since it will have to write each bit to one disk, then the other one, rather than to both at once. Don't ask me how you figure out if your can, I don't know other than consulting your motherboard/HD controller documentation.
Also, please note that almost /all/ so-called "BIOS hardware RAID" is NOT hardware RAID but software RAID provided by the BIOS. What's the difference you ask ? It's really a matter of encapuslation (done any OOP programming?). For true hardware RAID to work you don't need any special software because the RAID controller card does it all for you, making your array appear to the OS as a single device. Software RAID, on the other hand, relies on drivers at the OS level, 90% of which exist only for Windows XP, wouldn't you know it.
My 0,02 $ : on my servers I use RAID (software in the OS or hardware based on my speed needs), er and then `scp` my thesis documents to a remote server :P