McCoy isn't the same type of player Dareus is. Two totally different types of players. Dareus plays real low whereas McCoy plays more upright. Dareus is 6'3" and weighs 319, McCoy is 6'4" and weighs only 295. Dareus will be a better player than McCoy, IMO. He will be better against the run because he is heavier and shorter than McCoy, plus he has that lightning quick take-off at the snap, ala Kyle Williams, that will give O-lineman fits.
I can only imagine what it would be like to see Kyle lined-up next to Marcell. Our LBers will be free to roam, as well, as a result of the play of those two up-front.