The MR 223 is probably a bit better gun for your money. It has a heavier match grade (non-chrome lined barrel) which is press fitted into the receiver for greater accuracy. Being a Non-Military HK Product, you will be able to order barrels and parts ($$$), although they may take a bit for delivery.
I'm very happy with mine and it shoots very well. I have shot a few thousand rounds through Hk 416's and I think the MR 223 is more accurate, although a bit heavier due to the barrel.
If you plan on dive operations (in salt water, Navy Seal style) you might be able to take advantage of the chrome lining.
Rich