Lionheart, "Ringing" the bbls can show if there's a separation.
Take the shotgun down as if for cleaning or storing and...
Suspend the bbls by a wire, and hit them someplace around mid point with a pencil or other non marring, medium hard instrument. Listen to them. A clear and bell like ring means no separations, a thud means Trouble.
If you trust the dealer and believe he'll fix any immediate probs, skip the smith. I've a couple shops I trust, usually referred to here as Guns R Us.
As for the Guild gun, the few I've seen have been decent pieces,tho Germanic stocks don't thrill me.
All else equal, I'd get the Nitro. But, my Chauvinism about things US made is legendary.
One small caveat....
Old doubles can be a morass of problems. Tho ones like this were well made,but who knows what has happened to it in the decades since it left the factory. OTOH, I doubt you're going to run 5000 rounds a year through a piece like this, and it should(Fingers crossed and prayers said) do very,very well as an upland gun and for informal clays.
Also, most older shotguns other than Skeet guns are choked on the tight side. Modern ammo is way better than the old stuff, and you may want to open those chokes to something like IC and LM for the best use.
Oh,yeah...
I believe that Uncle Dan did some of the research and development on the first 12 ga mags, the Super Fox model in particular. I think one of McIntosh's books has something on this.
HTH....