The "old" A5's are a classic. The new A5's .... who knows what they will be viewed as down the road?
The primary difference between the two designs is the action. The old version is a rather complex, but ingenious design of gas operated movement of the barrel and bolt together, timed just so in order to accomplish safe and rapid auto loading. You can find more detail information posted on one of serveral web sites.
The new A5, in my opinion is a "copied" hump backed look with a whole different cycling design ... a Benelli like inertia system. This system is simpler, cleaner but results in heavier "felt" recoil.
For my $1200 +/-, I'd rather have a good looking used "original" A5 or a new Benelli SBE-II.