If a burner is DVD+R as well as being capable of the other formats, then the terms dual and double mean the same thing and they can call a drive either double or dual. If a drive is DVD+R only, then it will be labeled as "double layer". When it comes to the disks, however, its the same thing. They are simply disks with more 2 layers instead of one. i never really understood the need for the weird termonology, but someone thought it made sense at some point.
As for cheaper online, 30.00 isnt a bad deal really