Felix has a special talent for breaking off gigantic runs - but he isn't all feast or famine. He demonstrates better vision for between the tackles running than Barber does. It is not just Felix's talent rushing that we can exploit but the threat of him running is a much bigger threat than Barber and opens things up for everybody.
Barber is still an okay player but he looked like a shell of his former self much of the year. His balance just doesn't look very good out there these days and he is very indecisive when running the ball and requires a large hole to be opened to have any success at this point. He wasn't slow when he first came in the league but he wasn't slow either - he just looks slow out there now whenever he hits the open field. Finally there is the thing that you hardly ever see out of him anymore - that is running with power in the second level.
Choice is kind of the jack of all trades but master of none. He is a valuable player and may even be the best of the bunch in some ways - blocking and vision. He is also probably the best of the three runners right now in short yardage because he can squeeze into tight holes, jump over the pile or bounce it to the outside with equal efficacy.
It is as others have said the carries distribution that has got people irritated. I am not sure Garrett and Barber can co-exist on the same football team with another candidate for the top RB spot. Garrett wants Barber to have the vast majority of carries and snaps whereas I and many other posters (and probably Jerry Jones) think the offense is better with Felix in there. If Garrett could curtail his use of Barber to about 10 carries a game mostly focused to the second half, short-yardage and mop-up duty then I have no real issue with Barber being here. He can still be a useful part of the team.
However, on a cost efficiency basis, it is hard to argue about a duo of Felix and Choice. If there is no salary cap, there is no issue. If the salary cap comes back, Barber is the first one who should go because you no longer get much bang for the buck from him playing a backup/complementary sort of role.