They only real facts that we know really are quite simple...each bullet although equal in terms of energy...will leave very different types of wound channels.Shot placement being equal, can I assume that the 12 ga. by virtue of it's size would do more damage and after the larger 12 ga. slug expands it might do much more damage, thereby being more effective?
The 12ga would be less likely to pass through, and more likely to cause large amounts of internal damage. On the other hand the 20ga is more likely to pass though and less likely to cause a great deal of internal damage.
Personally, I believe the 12ga would be a much better projectile at that distance. The reason is simple, at 100yds you don't want a deer running off a greater distance and having to follow a bloodtrail. The ideal scenario is one where the deer is dropped with the first shot. The more internal damage that occurs, the more likely an animal is to drop on the first shot. That means to me..the 12ga would be the prefered projectile.