Have I backed up to wrong media?

[Hotdog2]

After using my PC to play dvds for sometime, I finally decided without looking into it too hard, to buy a set top.(today)
Trouble is it wont regonise the "Backup" dvd+r im sticking into it.
So looked in the manual and it states it will play dvd and dvd-r.
My backups were tested on a friends cheap and nasty set top
and worked fine.
Whats the best course?
Do I take back the set top and find one that plays +r or should I be making my backups to -r.