If in case you are not too familiar with what these things are, these would refer to different guns. Some would like these two models in particular because of their many features. The MR556 is considered to have a superior barrel, especially when you would compare it to the HK416.
The chrome lining is definitely going to be important. The HK416 comes with a free-floating four-quadrant rail system that can actually become very effective, especially when you would like to make use of different accessories that can help improve the way that you use this type of gun. The MR556 comes with a lot of newer functions that can help you use the gun better.