Richard,
Lots of competition cookers inject due to timing constraints. Some event organizers provide butts to the cook teams, and usually very shortly before they need to be on the pit. This timing issue forces injections to be used instead of brining.
I have tried both methods, and feel they each offer their own advantages.
Injections are quick and can offer ways to get flavors deeply into the muscle tissue.
Brining gives good tenderness and better moisture retention, however, butts are very naturally moist anyway.
Experiment by doing one of each and cooking side by side on the WSM. See what works for you, and which flavor and texture profiles you like best.