The DVD minus and plus formats were created by two different companies but to be honest there is not that much difference.
The important thing is to check which format your hardware (PCs, laptops, DVD players etc support).
Nowadays most hardware supports both minus and plus but some older hardware does not so dig out your paperwork and see what they say they support.
One word of warning. Files can be deleted off a RW disk, so they are not the best for backing up files.
I always back up to "R" disks as once written to the files cannot be deleted.
I tend to use CDs for this rather than DVDs as it can get expensive using a new DVD each time you want to back up pictures.
I also do 2 copies of each because I have had some CDs or DVDs that get corrupt and it is good to have a "spare" backup.