It used to be that the Vepr was the "De Luxe" AK shotgun with all the bells and whistles.
The Saiga needed a pistol grip conversion, 922r parts, rivet hole plugging, and maybe some refinishing after all that. And was a little more fickle. It made up for that by costing half that of the Vepr.
However, Obama has banned the Saiga series from import, so now their prices have gotten very silly. And on the other hand, current Vepr 12 prices are also about as low as they've ever been.
Unless the seller doesn't know this, or you are interested in collector's value, the Vepr is the way to go.
The DDI-12 might also be worth investigating.