Not sure what 'nicer' means, but you put it in quotes like I did. Seems to me with CO2 you will broaden the pen pattern and reduce the oxidation potential. This should manifest as cleaner looking welds (maybe), fewer surface deposits(maybe), but of course more Si and Mn in the weld metal as well(logically). You should also notice reduced edge wetting, and higher voltages.
If your guys are running close to transition you may lose spray transfer, or at least destabilize until you increase current. But a slight increase will get it back. You may see more spatter at first.
Mechanically I'm not sure it makes much of a difference. Probably is but I doubt its outside the normal scatter band. I'd go with cost on this one. We use 95/5O2.
My opinion.
You could use 80 to 90% Argon with the remainder CO2. This would allow short circuit spray and pulse spray transfer with one gas. Make a decision what mechanical and physical properties needs to be achieved and test the wire, gas and material combination. ie qualify the procedure, a good test is worth a thousand opinions.