The optimum speed that the DVD can be written to. Check with your DVD burner to see what speed it can write to because if you only have an 8X DVD burner there is no point in getting a 16X DVD.
True. In fact, some older SuperDrives actually performed worse if you didn't use the media specified for the drive. I think it was the 4x drives that would only write at 1x if you used the 2x media. I suppose they've fixed that problem, though, as it's been several years since the problem first arose and I haven't heard much about it in a long time.
The optimum speed that the DVD can be written to. Check with your DVD burner to see what speed it can write to because if you only have an 8X DVD burner there is no point in getting a 16X DVD.
True to a point.
If they cost the same it doesn't matter which you purchase.
If the media is rated at 16x and your drive only writes at 8x, the drive will automatically burn the discs at 8x (or, the fastest speed the drive can burn) and you shouldn't have any problems.
For instance - I have a DVD/CD combo burner that burns DVD-/+ at 8x and burns CDR at 48x. Nowadays all the DVD-R media I find is 16x but I buy it anyway because my drive will burn it at 8x and I've never had a problem yet. (I don't buy 'plus R' media ie DVD+R because my standalone DVD changer does not like it.) For music CDs I buy Music CDRs because most of them (the ones I buy at least...Maxells) have long term dye whereas standard CDRs have short term dye (and therefore have more problems in standalone CD players because the short term dye is thinner and cheaper to make). Anyhow - every Music CDR I've used only burned at 32x because I guess that was the cap they hit on the Music CDR burners that stores sell, so my 48x drive only burns them at 32x but (again) - no problems.
The point is - most media sold nowadays is rated faster than the drive you have in your computer, unless the computer is brand new or you just upgraded the drive. No matter what you buy, as long as it is rated as fast (or faster) than your drive, you'll be ok. Don't spend extra money on harder-to-find 8x rated media because you think 16x won't work, because it will.