If by well built you mean reliable and durable the answer is there is no difference between guns built by the company named UA or Grulla. Neither UA nor Grulla made the parts they used to assemble shotguns. Like almost all other makers at almost all other times, UA/Grulla bought/buys the parts and assemblies needed to assemble they shotguns that will have their name upon them. If there are differences in the materials used to make the parts AU used, from the materials used to make the parts Grulla uses, its because time has passed and the network of suppliers UA/Grulla buys from has changed the materials they use to make their wares.
Concerning whether the people of UA or Grulla did a better job of assembling those parts into shotguns, remember that while the company name changed the people did not.
Im getting the sense that you are new to Spanish shotguns, and are looking to make a first purchase. If thats the case my best advice, offered FWIW, is:
1) Ignore the names on the guns and just look at the gun and then buy the gun that best meets your needs, regardless of who made it. Youll pay less for the same gun.
2) Buy a used gun rather than a new gun, especially a gun made roughly during the period 1945 to 1975. That was the period that skilled hand labor was at its cheapest, and more of that skilled hand labor went into guns then than now does into guns at any similar price point. Youll get more gun for your money.
