Master Grade tapes generally have a more archival quality to them, meaning they have a higher resistance to degradation than standard MiniDv tapes, and are therefore used for mastering (or putting your final project to tape for archiving).
DV is DV, so the footage is recorded to the tape the same way, no matter what. The difference, I believe, comes from the quality of the lubricant on the physical tape that is used. There is a special lubricant on the magnetic tape in a cassette. This lubricant degrades over time and will cause dropouts and hits over your images. Standard MiniDv tapes last around 7 years before significant degradation starts to take it's toll (and only when properly stored). I understand that DVCAM tapes will last around 11 years in storage. As far as the Master Grade MiniDv tapes, the archival length probably depends on the manufacturer. Check your tape manufacturer's website for information on this.
Chris
