Seawater injection gives you as much volume as required, as this is abundantly available. Well established technology, few issues with operating the SWI system.
Produced water re-injection is limited to the volumes available, which may be inadequate sometimes. Also, solids issues are pronounced in this case. PWRI reduces environmental discharge.
Scaling and souring need to be evaluated separately in each of these cases. Need for scaling control in WI systems as well as prevention of souring in reservoirs are critical to ensure maximum productivity.