We do a lot of prime rib at the joint during the holidays. Most of them run close to 20lbs. I smoke them around the 250 degree range. Why? Two reasons...since this cut of meat is pulled off the smoker at a much lower temperature than a tough cut of meat (brisket, ect.), it won't be on the smoker nearly as long. At around 250 degrees, it will cook long enough to develope a good smoke penetration. Second reason is that this temperature is hot enough to create a nice bark for the amount of time the prime rib will be on the pit, if you don't pull it before around 140 degrees internally. Most of my customers want it cook to around 140-145 internally. If I pull them at 140 degrees internally, they will generally rise to about 144 degrees before the internally temperature starts to drop.
Remember an important tip when trying to cook for a crowd. People that like their beef rare will eat it if it is a little past that point (not burned up!). However, people that like their beef done will not eat rare beef (most of the time). Just something I've pick up over the years.
Danny