It really depends on the ...

It really depends on the water you are screening, the downstream processing units, and ability of you to dispose of sludge.

Ferric will usually create a greater volume and mass of sludge to be disposed of compared to alum, but can do a better job of removing TOC components in the raw water.

Both chemicals consume alkalinity in their coagulation activity so expect significant pH depression and need for post clarification pH adjustment. If you are looking at demineralizing/ion exchange after the clarification, both will add significant ionic loading (especially considering the pH adjustment needed) and shorten your run times.

There are coagulants that do not produce the amount of sludge these metal salts do and clarify equally as well. They also make coagulant blends containing ferric or some aluminum salt paired with an organic coagulant that will help reduce sludge production while still maintaining the clarification and TOC reduction desired by the metal coagulants alone.

Bottom line, jar test and see what works best.