There is a difference but it isn't huge. The m4 2000 will be quieter. The numbers I have seen is the 762 SDN will do about 138 DBL on a 556 and the M4-2000 is like 133 or so. This video may help. It doesn't have the M4 2000 in it but may give you an idea.
http://youtu.be/XWo0JGqWw7w
I really can't tell an appreciable difference at the shooter position.
One day I'll have to get someone to shoot them while I stand off & see what they're like.