To add to Lawrence's reply (and I think Lwrence helped me out last time too;-) we use 321 and had a lot of problems, these were traced to bad prep and a faulty argon regulator.
With 321 I cant stress enough the pre weld cleaning (every one else stressed this too;-), I tried scotch bright but found it still left some oxide, coudn't see it and the surface looked good enough to weld but if we use a fine flap wheel to clean first the result is a much better weld at lower temp settings, so I assume a little oxide was still left on the parent metal.
The second thing was getting the purge right, again I have found this makes a fundamental differance to the weld quality and the way the weld pool behaves, all this I learned from the forum as I am only a novice myself.
From experiance i can now say if you get the purge and prep right the differance you will find is like chalk and cheese!
Hope that helps