The latter is probably the entirety of why the Chief's armor was changed. They use motion capture for the Chief and other characters in Blur's H2A cinematics, yet in those the Chief's armor is a very close replica to how he appears in-game. I got around to watching some of them over the weekend, and it shows that they could have left the Chief's armor more or less untouched from how it looked in Halo 3 and still used mo-cap to get all the animations they needed.