Where are you getting those stats from? I presume you are talking about the 2011/2012 season which is when they were both in CM in the PL, and every stat site I'm looking at has Cabaye with 5 goals overall in English competitions and Fellaini also on 5...
Plus Cabaye takes free kicks doesn't he whereas Fellaini does not. That added 2 or 3 goals from what i can find.
On your point about the winning the ball and tackling, i understand that, and it's where Fellaini takes over Cabaye in that particular department.
From EPL Index.
In that particular season, Fellaini had 84/99 being successful. Cabaye made more tackles so understandably would have more successful tackles, but Fellaini had a higher percentage of his tackles being successful, around 85% i believe. Fellaini also won the ball more than any player in CM during that season too...
More useful defensively because they win the ball, well yes, which is why Fellaini is so good and doesn't need to be an overly brilliant passer or of particular creative talent. He is more than good enough to link play up without making the 'key passes' but within United he wouldn't need that as he has Carrick beside him. Cabaye to me is like a worse Carrick, whereas Fellaini is a midfielder that could slot beside Carrick, do more of the dirty work and free Carricks game up a bit more. Plus the benefit of that is when you need Carrick to sit out, you don't need to worry about losing much of the defensive part of the midfield anymore, you just need to worry about creativity which Cleverley/Anderson can give you. Plus Moyes said he wanted a defensive and a creative midfielder IIRC.