The fx991 series is quite advanced comparatively and could do many things the fx-83 cannot. Comparison of Casio scientific calculators could be basically done through considering the number of functions the calculator can handle.
fx-83es: 249 Functions
fx991ms: 401 Functions
fx991es: 403 Functions
fx991es plus: 417 Functions
But still this may not tell you all the good and bad qualities. I suggest you to read the manuals of each calculator (Calculators - Manuals - Support - CASIO) which will give you a good picture.
All the fx991 series calculators do not differ significantly in their abilities. Although there are minor improvements and upgrades, most of the improvements are to increase the user friendliness. So almost the same things could be done using each calculator.