Even though the DVD specification allows double sides, there are no publicly available DVD+Rs, DVD-R, or DVD-RAM discs on sale which record on both sides since it's cheaper to manufacture two one-sided discs.
The CD specification does not allow double sides, on the other hand.
The difference between DVD+R and DVD-Rs are subtle now that most recorders and players support reading and writing of both formats. DVD-Rs are slightly more comaptible with regular DVD players than DVD+Rs, but only by a few percent. Some players write one format faster than the other, depending on which alliance the manufacturer sided with.
Of course there are always heated discussions on whether the DVD+R specification or the DVD-R specification is better. Yet it's mostly the case that the quality of the Disc maker and the writer device determines the life and reliability of the burnt DVD so I wouldn't worry too much about which to pick.
DVD+R discs do allow double layer recordings, though. Double layer recording takes place on 1 side but there are two layers of recordable platters so that 9.4 gigabytes can be written to a disc. The newest DVD+R writers support this but there aren't any widely avaialble and cheap double layer discs out yet.