I found with my sealey mightymig 150 i was struggling to get penetration on 2-3mm steel on 0.6 wire. Soon as i switched to 0.8 i was blowing holes in it fairly easily. So 0.8 works better for me on thicker stuff, sure one of the experts will be along soon with some advice.
I would go for the 0.8mm personally, With my 185A I struggled to fully penetrate 2mm with 0.6mm wire, although for a lap weld or plug weld then the 0.6 might be ok as you are not looking for total penetration as per a butt weld.
Depends on the application I suppose, what are you welding?
i tried it last week on a 2mm plate and i can get the 0.6 to push all the way through with best part of a 2mm buldge on the other side.
all im trying to weld is some old industrial shelving to turn it onto a log store. so far everything has been bolted together but i need to add some plates for the roof to screw to, its nothing really structural.
think i might just go get a reel of 0.8 anyway