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Apple (and Pioneer) have traditionally preferred -R, to the extent with some drives that they will only accept/mount/burn -R discs. This has caused issues for me with playing DVD-R discs on a PS2 - frequent fatal errors because Sony prefers +R (although not so much with my Benq burner, almost always with my LG burner). Other generic players had no problems with them tho and i ended up getting a Pioneer DVR - no problems at all now.
I presume the MBP still uses Pioneer drives, with the same issues.
 
The MacBook Pro says it can handle + or - , so either should be fine. I have a Pioneer SuperDrive and haven't had any problems, but most of the time I use -R because I believe it is a little more widely accepted.
 
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