Have Fast, Mixed Sets use -SpD over -Def

[Tmi489]

Most priority moves are physical, so losing defense over special defense doesn't make too much sense. If securing a KO, being able to take a priority hit is more important than a slower one.
If Def=SpD, then -Def should still be preferred in order to give the Porygon line +Atk on Download.

 

[sumwun]

I have heard that extra special defense is more likely to save a Pokemon. I'm not sure how credible it is, though.