Whichever you choose, some day you may wish you'd chosen the other.
On my machines, it's OFF. I have all the install disks for the software (or the downloads burned onto CD or DVD). The data files get backed up regularly (apart from the ones where the data is hidden by software and I haven't sussed that yet).
Most important stuff is on the desktop and the laptop, just in case.
I did use it, years ago. When the machine was zapped by a virus, the System Restore failed anyway.