Well, from a user's perspective, the only real difference between + and - is maximum speed. The + seems to burn a little faster. There may also be a price difference.

The - discs also can work on some older units that won't play the + discs. But since this is an emergency backup, buying a new $30 DVD drive would not be much of a big deal if the computer that you were trying to restore to could not read it.

BTW: You might want to re-think how you store those discs. Thy dyes on recordable discs change color based on heat. In the event that a fire safe has to stand up to a fire, the disc might not melt, but there still might be enough heat to ruin your data. You might also want to consider sending periodic copies of your discs to a trusted friend/relative who lives in a different part of the country.

**EDIT**
You might also want to consider a hard drive for backup. You can get a 2-1/2" hard drive in a USB enclosure. You plug this into your computer, and copy your files over. When done, unplug and put it in the safe. It would be rather large, but would allow you to keep 100 gigs in a box 1" x 4" x 5". If space is not a problem in your safe, you can store 500 gigs in a box less than 2" x 7" x 10".