The Saiga's as imported and sold by most places is not the same as the current Arsenal Inc.
What Arsenal Inc gets is basically a special Saiga barreled action that's custom made just for them by Izhmash, who also make the standard Saiga.
The Arsenal rifles don't have the holes in the rear of the receiver for the Saiga standard rifles for the trigger group.
The Arsenal also has the full military front sight and muzzle brake assembly and the military hand guards.
These have the 1000 meter rear sight.
The Arsenal accepts all AK-74 magazines.
The standard Saiga as in the ad you posted have the two rear holes plugged with rivets and commercial hand guards, and no threaded barrel for a flash hider.
They have a 300 meter rear sight.
I'm not sure about that particular conversion, but the standard Saiga requires or at least used to require a special magazine and you can't use military mags without installing a riveted bullet guide.
Bottom line: The rifle in the ad is a minimum/partial conversion of the commercial Saiga.
The Arsenal-Saiga is a full conversion to military form using a special barreled action that has more of a military setup.
They will shoot about the same.