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Should be the same thing but it may be cut into smaller pieces. Butt or shoulder make little difference in flavor. Here on the west coast we have picnic shoulder as the most common cut available. I can get Boston Butt at Sam's or Costco with no problem but not so much as the chain grocery stores. They all smoke the same and pull just as well though. If it's on sale then my only advice is buy as much as you can freeze :)
The shoulder as a whole is compromised of the Butt/shoulder and Picnic/shoulder.
The Butt is the upper half of the shoulder and the Picnic is the lower half but above the shank.
What is usually sold as a Shoulder or Butt are basically the same cut, sometimes some differences in the way they are trimmed.
They all cook up great and make nice pulled pork.