They are both known as "stabilized" SSs. 321 has a bit of Ti which combines with C easier than Cr. 347 uses Nb for the same purpose--to prevent Cr carbide formation, which removes (denudes, as they say) the areas right next to grain boundaries.
347 works well post-weld as is, but 321 benefits from a moderate-temp. heat treatment, IIRC.
The first "stabilized" grades of S.S., 321 and 347, are generally used at operating temperatures exceeding 800 F. 347 has somewhat lower creep ductility than 321 so at service temperatures above 1000 F, 321 may provide greater life without cracking in a bellows application.
