There are distinct additions to Stephanie that Victoria does not have.  The shape of her ribs, waist, hips and abdoman for example- they all can be greatly changed and enhanced via Stephanie's morphs.  Not to mention additional musculature changes in her legs and arms, as well as depth changes in her shoulders, lower back and upper thighs.

 I use both Stephanie morphs as well as David; there were benefits from having D3's morphs placed into M3 that really gave you a well rounded character when used.  Now with Genesis, it's even more obvious.  

 The more options, the more unique a character you can make.  I am one that chooses unique and options rather than the greater normalcy, but in the end it is up to the purchaser.