Basically they are 2 formats that do almost the same thing.
I am not going to go into too much technical detail because I don't know how much you want to hear that, but there are 2 reasons I use dvd+r now. One is that it has better error correction so it will withstand scratches better (dvd-r only error corrects around 2/3 of the info on the disc while dvd+r error corrects all) and you are able to set the bit type of the disc to real dvd-rom. Basically the bit-type is what tells the player what type of disc it is. While in the past some players couldn't read dvd+r's it was because they didnt recognize the bit-type. If you have a good burning program you can usually set the bit-type. If you can set it, then you can make players think it is really a pressed DVD and actually have better compatibility. However, I think all players made in the last few years read them fine.
In short, I would use dvd+r's because of the extra error correction. In fact, I don't even bother setting bit-type anymore.