A single action bar because the barrel is threaded into the receiver on the Model 12 and Model 31 and Model 17 and Model 37, its a stronger hook up between receiver and barrel so you don't need dual action bars.
http://www.randywakeman.com/Ithaca37_Browning_ BPS_Compare.htm
The way I see it you have to look at the Model 12 and its stellar history. That damn shotgun excelled at everything it did. The American gold medal winning Olympic trap team in 1920, Model 12s were used by some members. The 1924 American gold medal winning Olympic trap team all members used Model 12s except one who used a double, The Individual Olympic gold medal winner in either 1952 or 1956 was a Canadian who used a Model 12.
You may say, so what, Perazzi and Beretta are dominating today. But now you look at its military record. The acclaim in both World Wars plus our military ordered more Model 12s in World War Two than any other shotgun and its use is documented throughout the Pacific Theater. You don't see to many Perazzi shotguns on the battlefield.
In my opinion, that is documented with facts that are easily verified, there isn't a shotgun made on this planet that can match its history.
But its now 2017 and its old. Some say they use steel in their Model 12s but I've never seen it and according to the last story I've heard all Remington 870s are steel capable. If you have to use other types of non toxic shotshells they're very expensive in relation to steel, which can be used in the Remington you're looking to buy.
Its your decision but with this lead shot controversy it may get worse or other states may adopt California's draconian laws. Then where will you be 10 or 20 years down the road?
) but I like the advantage twin bars seem to offer. 
