Get a dual layer recorer! If you don't you will have to open up any DVD you record and "shrink" it down before placing it on a DVD R. However, dual layer discs are very expensive at this point. three to four dollars per balnk DVD!
The - and + mean little except that some home DVD players will play one or both. Avoid DVD RAWM because no one uses that standard at all.
Are you going to back up DVD movies? Or are you just looking for the ability to record data discs?