Steel hull zincs - screws or welding

No, you want to be able to change the zincs in the water. Not have to haul the boat to change a wear item like zincs. As long as the bolts are making good contact, they are fine.

Bolts are way better. Welding is a pain especially if the opposite side of the hull is not easily accessible . Recommend you chase the threads on the studs and use some marine grade never seez and lock washers when you bolt on new zincs. If you are blasting and painting the hull just cut some short pieces of an old hose that fits snug on the studs and that will protect them while you blast and paint.

I deal with weld-on zincs regularly. They are very typical on commercial, steel hulls. The zincs come with steel tabs to weld to the hull. But as mentioned, bolt-on are much easier provided they are done correctly.