DVD-R DL is dual layer and DVD+R DL is double layer?
dual layer = double layer. Both are 9.4gig (4.7g x 2). It's the +/- format that makes them different. It's not too much of a concern if your dvd reader can read both formats. It's more of a problem with older set top DVD players for television. I think all DVD players can play the DVD- format as that was the original DVD spec. DVD+ came out later and is not supported by all players.
Someone correct me if I'm wrong.