The strap you should use depends on the pain you are experiencing. If your pain is occurring from your knee down, an infra-patellar strap is probably your best choice because it is your patellar tendon that needs extra support due to an injury.
If your pain is in your knee but does not go below it, or if the associated pain is occurring in your thigh, a supra-patellar strap is most likely what you need because it will support your knee and quadriceps muscles, which help stabilize and move the knee.
If you aren’t sure what exactly is causing your knee pain or don’t feel confident in selecting a strap or identifying the area that is affected, your best bet is to schedule an appointment with your physician or physical therapist.