Sephiroth's mobility might be slower in Bizarro or Safer form (I doubt it would be though in Safer) but still his defense is much more stronger. I mean just compare hit points between Sephiroth and Safer Sephiroth.

Sephiroth: about 5,200

Safer Sephiroth: about 72,000

and if all of your characters are maxed out, then Safer Sephiroth's hit points get extremely higher as a result. So any way you put it the two forms would help Sephiroth out that is if we are using them.