the dvd-r vs dvd+r is kind of like betamax vs vhs.
It's kinda funny to use that as a parallel. From what I've read, Beta was a superior format in both picture quality and audio capability but we all got duped by the marketers.
Everyone wanted to buy a vcr in the format that would win the war...somehow they convinced us that was VHS and then it was like tipping a boat with water in it. The more it tipped, the more water rushed over to the other side and pretty soon it was decided
In this case, I don't think there are any quality differences - just compatibility. I also bought a dual burner and will probably end up trying all kinds of media and end up with whatever works at the right price.