The difference is how fast you can write the data to those discs. If you have an 8x DVD Burner, you can buy either speed of media, but 8x media will not work with a 4x burner.
You should also take note that DVD-R and DVD+R are different. It's like the old VHS and Beta wars of the 80s. Chances are that your burner (if we're talking about a computer drive) can write to either kind, but your standalone DVD player (ie. one that is attached to your television) may only be able to read one kind (if either!).
You should check the manual of your DVD player to find out if it supports recorded media, and if so, whether it's + or -.
All that being said, don't be surprised if the copies you make won't play in your standalone DVD player...even if you're sure it is compatible. The art of DVD backup has yet to be standardized, and success rates are highly variable.
Good luck...and you might want to check out videohelp.com. You can look up your specific DVD player to see if other people have had success with it.