Welcome to the group! If you're running adjustable upper control arms, both should be adjustable. They are for adjusting pinion angle, if I'm not mistaken. They may be needed depending on changes to your suspension and drivetrain. If you're fairly stock, I doubt you'll need them. If you are slammed and plan on racing, you will want fully adjustable rear suspension (that's my understanding, so someone may chime in with better info).
My main issue with aftermarket upper control arms is that they don't allow for any flex. If your upper torque boxes aren't reinforced, you risk damaging them with hard launches. If you don't need adjustables, just run stock uppers and aftermarket lowers. Or my favorite option... use the upper torque box reinforcement plates from Wild Rides and use stock upper control arms that have been boxed in (just weld a sheet of 3/16 or so steel across it for reinforcement).
Of course, you'll get different answers on this topic but I've found this works and it's inexpensive. Spherical bushings for the upper diff housing is another mod you can do to address the inherent problems.
