321 is a Ti stabilized SS, and that of course is the reason for Ti in 316. Normally you would weld 321 with 347, but in a dissimilar with 304 SS 309 is fine, and with a diss with 316 I would say 309 is fine as well, though with the 321/316Ti combo you have two stabilized alloys whereas 309L is not stabilized, though it does have low carbon. 309 would certainly be less crack sensitive than 347.
If the stabiliztion is an issue (though your temp is well below any service sensitization range-and since the weldments will most likely be as welded) on the 321/316Ti combo run 316Ti filler. Again, it will be less crack sensitive than 347.
Its an engineering and availability issue.
In general, the choice of 309L for joining two different types of stainless (in the 3XX family) would not be my first choice. 309L is a good choice for welding of 3XX series stainless to carbon or low alloy steels, but the overalloying, provided by 309L, is wasted when both base materials are in the 3XX family (unless one would be 309 or 310). If you match the higher of the two 3XX base materials, your results should be satisfactory. An example, 304 to 304 would normally be welded with 308(L). 304 to 316 would normally be welded with 316(L). In all cases, the end use (corrosive environment, temperature, etc.) of the base metal combination needs to be understood when selecting the appropriate filler. 309(L) normally can be used, but may not be the best choice.
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