Hi Dagwood
The big difference between 304 and 316 is that 316 contains around 2 - 3% Molybdenum. Mo helps 316 to have a better pitting resistance than 304. In a service where there is a possibility of pitting corrosion, 316 would be a better alloy to use than 304.
Alloy 304 is however better in oxidizing acid service, such as Nitric Acid. It is also somewhat cheaper than alloy 316, so it is used where the pitting resistance of 316 is not required.
It is generally not recommended to use 316 filler to weld 304, because the ferrite content will probably not be what it should be. In addition, if you need resistance to oxidizing acids such as Nitric acid, the weld metal will perform poorer than the base metal.
In many alkaline environments, S/Steel is not required, because these environments tend to be non aggressive to C/Steel, so why pay more. This is however only a basic approximation and should not be taken as being a hard and fast rule.
Hope this helps.
Regards
Niekie Jooste
Fabristruct Solutions