It all depends on the application.
IF a little sensitisation wouldn't hurt then 304L should be fine. But if corosion (pitting or CSCC) are concerns remember that they are both accelerated by sensitization. In that case the 321 would have some advantages.
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Corrosion never sleeps, but it can be managed.
In comparison to 304L the higher carbon in 321 also provides better high temperature strength if that is an issue. My minimum creep data shows that at 1100F the Average stress for 0.0001%/hr creep rate of 304L is 7,700 psi vs. 20,000 for 321.
This is a screwed up thread. One imprecise question has resulted in no clear course and the poser of the question hasn't bothered to clarify. Please, no on respond further unless DVNMech can give the appropriate details.