A short while ago I raised a question about wire burn off factors:
http://forum.weldingtipsandtricks.com/v ... f=6&t=3480
I did some home work on this and I have not noted the source but read that since the amps are carried to the arc (if that makes sense) by the metal cylinder that is the wire for flux core wire, the more metal in the cylinder, or tube around the flux, the more amps that can be used for melting steel.
OK, if that is so, then .035" (.9mm) wire will enable you to weld thicker steel, and, at some point, that will be a critical difference between using .03" and .035" wire. (If we accept the theory outlined above.) The appearance of the weld may not be different, but in the "doing", would you agree with the heavier wire you can weld hotter and faster?
Also, it would be interesting to use your question (does FC diameter make a difference?) to model some performances with the Miller calculator (if that has general application, and is not specific to Miller technology).
I have standardised my welding, as much as I can, and try to use .9mm, all position, limited multi-pass wire for my FCAW fabrication work. Because this covers a wide range of material (I only do "light fabrication", really), I seem to be getting away with this for the past few years.
Incidentally, it is amazing how robust a construction one can make with C section, or SHS / RHS, with 100 or 120 mm cross section and only 4 mm wall. In a few metres of length, that is a lot of steel, in my "light fabrication" field work.
