Should I use PID or PIV?

This is a very commonly asked question. There are several things to consider when answering this question. First what are PID and PIV?

PID is a control algorithm based on position error.
PID is a very common control algorithm in industry. It is used for systems as varied as precise motion control to fluid level control to temperature control to ..
PIV is a control algorithm based on velocity error for its inner loop and position error for its outer loop.
PIV is similar in concept to a velocity drive that has a position loop around it. The XMP implements the velocity loop and position loop inside its DSP.

Here are some things to consider before making the decision between PID and PIV:

Are you already comfortable tuning one method? If you are comfortable with one, you likely will get better performance and reduce your tuning time significantly.
Many more people have experience with PID tuning. If you have experience tuning systems with tachometer feedback, you may find PIV more familiar.
Try both. If performance is critical to you, you should explore all reasonable possibilities. Trying PID and PIV is not an unreasonble amount of effort if performance is critical.
How linear is your system? PID has good results if the system is a simple mass with a force or torque transducer (i.e. simple permanent magnet motor) powering it. If you have a robot with linkages that change mass moment of inertia when it moves and an exotic motor that is not a perfect torque/force transducer, PIV may work better.
Systems that care more about velocity regultation than position error will likely have better velocity regulation with PIV than PID.
PIV has more parameters to adjust than PID. This is a pro and a con.