Horford at his peak was better than Toine, but we aren't getting Horford at his peak.
Personally I think Horford's decline is exaggerated, his numbers have more to do with his situation and IMO will go up this year in Boston as the front court focus vs playing next to the fantastic stat stuffer that is Millsap.
You bring up 'Toine's good VORP numbers, but there are some very ugly ones too and Horford's are consistently good.
To the eye test (still all important on defense IMO where stats are lacking), it's really not close as defenders. As well as the fact that Horford is a legit big who can defend the paint where Toine was a tweener type and certainly wasn't locking anyone down on the wing.