IMO (eye test)
Drummond is better team defender.
Whiteside is a better ISO defender.
Just look at who Whiteside has covering for him.
Dragic, Wade, Deng and Bosh who are all above average defenders.
Drummond has nobody....except for maybe Pope.
If I had to pick who I'd want next to AD I'd go Whiteside, but only because there is about a 5% of that happening compared to Drummond at 0%.
Oh and Drummond is ******** house at free throws and you can't play him at the end of games.