For me, it's DVD-Rs because my DVD player reads DVD-Rs and I prefer to watch them on a large screen.
The bottomline line is if you are planning to play it on DVD players, check which formats it reads. If you're playing it on your PC-DVD Writer, either one will do. In addition, do note that not all DVD+R/-R medias are made equal ... there are more reliable brands such as Fujifilm (which I'm currently using).
I am just guessing but I think DVD-R is for backward compatible with some older DVD players. I had a client who had trouble playing the slide show as I gave them a DVD+R. So I had to buy some DVD-R to reburn the slide show and it works brilliantly!
Hommie will probably have better answer.......! Yoh Uncle Hommie, HELP!