@Beepster: Slowing down pitch to half speed but maintaining the length means having to discard half of the samples.
Then, recording my voice at that pitch and doubling the pitch but maintaining the length means "inventing" half of the samples in the new version and guessing what level they should have. At 44k1 sample rate, I will have an audio bandwidth of around 12kHz
If the length is allowed to double, all those samples stay in existence. No loss.
And afterwards, when my contribution is pitched up and the length halved, no additional samples have to be "invented".
Incidentally, I use Melodyne externally only (I got a full license). The original artist's voice is an alto female voice. Pitched down by -9 steps to match my range makes it sound rough and growly. To get a good ghost vocal, I naturally have to try simulate what I am hearing, then pitch it up again.
Using a round number like -9 is indeed better - a half tone up or down makes no big difference to match my register range.