Well in the states there was an assault weapon ban in which particular features of firearms were not allowed to be used. Bayonets, pistol grips, etc.
So pre-ban, would be probably the better rifle in terms of features only. So it would probably have a removable flash hider, a bayonet lug and a hi capacity magazine (doesn't really matter cause in canada we are limited to 5 rounds).
Post ban is something made after the ban, I've heard that even though the ban has been lifted some of the features such as the bayonet lug are still not available.
Just some thoughts, I'm sure someone here will clear it up fully.