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We have found that large blacks are great backyard pigs. They let you handle the piglets, as long as you remember to give big momma a scratch too. Easy birth and super docile. Now I should mention that this is just my experience from visiting a neighbors farm, but the pigs all let me handle their young and I am a stranger. Also the berkshire are tame. We are breeding the 2 together for meat interest. I have a pregnant large black sow and she lets me rub her belly, check her temperature, even look at her teeth. The boar which is a berkshire is so gentle, he lays down for belly rubs as soon as you touch his side. We raised them both from babies, and because we gave them daily attention we have no worries. When our electric fence went down for 3 days, they never left the property. They did keep trying to come in the house though, ever try to stop a 300 lb sow from coming in the house door with you while you are carrying young children. They would always go back to the barn at night, but during the day they thought it was fun to beg at the door. My mini dachshund was very worried she would be sat on.
I think it's very much in the handling. In my experience, Berkshires were much more aggressive than other breeds, so aggressive as butcher pigs I never wanted to keep one as a breeding sow. The gentlest sow I ever had was a Duroc, the meanest sow I ever had was a Duroc.
Edit: Should have added, the sweet sow was one I raised from a weaner pig. The "mean" sow was one I bought when she was already 3 I think, and she was mean when I got her. On the other hand, she had consistently weaned 16-20 piglets, all heavyweights at butcher time (and continued to do so for several years, well past regular productive age for sows).  Too bad I hadn't gotten her as a weaner, she would have been a nice sow with big litters 
I agree the Yorkshires (the sows usually used in commercial breeder) tend to be better mothers = more protective. However, if you raise it from weaner size you can still have a sow that's easy to handle, regardless of her breed. 
I kept a Hampshire Boar (total sweetie), a Landrace Boar, some Duroc, Hampshire, Yorkshire, and cross bred sows. I sometimes would buy other breeds for butcher just to compare.
One thing to note - Berkshires have the best fat. Bacon and hams are incredible. The meat is very tender.
For feed conversion to the table (incl best butchering weight vs. live weight) Duroc/Hampshire crosses lead the pack, with Hampshire/Yorkshire crosses coming in second ("blue butts"). If you're going to keep your own boar, I'd rather have a Hampshire boar, and keep Duroc and Yorkshire sows.