They are staying within 2 degrees so far.It's nice that both pieces of meat are cooking at the same speed. I love it when that happens.
Those are the only real different there is. Because the shoulder is leaner if you try to cook it fast you are going dry it out. The butt because it is heavily fat marbled is very forgiving. I have cook them with high heat and have them cook in 4 hour for slicing and low and slow for 12 hour for pulling either way they are both delicious.
Shoulder, the meat is leaner,
Butt, heavily fat marbled,