If both CPU and PSU are available for given version which one, you will prefer to apply?

Once you apply the PSU then the recommended way is to apply the next PSU only. In fact, no need to apply CPU on the top of PSU as PSU contain CPU (If you apply CPU over PSU will considered you are trying to rollback the PSU and will require more effort in fact). So if you have not decided or applied any of the patches then, I will suggest you to go to use PSU patches. For more details refer: Oracle Products [ID 1430923.1], ID 1446582.1

PSU is superset of CPU and CPU mostly deals with security issues where as PSU fixes security issues as well as functionality changes.
if you are only concerned about security issues then CPU may be the good approach.
Oracle release both the patch sets every quarter.