Why is it a limited slip
 

Is the limited slip better than the positive attraction. Why

Same thing. Different names.

I believe you refer to the Ford Traction-Lok and the GM Positraction differentials.

Yes, they are two versions of the same thing, generally known as a 'limited slip' differential. All V-8 Mustangs (starting in the mid '80's) have this option installed.

Both the Ford and GM units are similar in design and operation and about equally effective in equalizing the rear tire grip. Both are servicable.