I prefer 0.8 as I find it more flexible with the power settings, you can wind power down for thin metal and wind it up for heavier sections, you can also do most chassis sections with it.
First welder I bought about 40 years ago had a roll of .6 wire and I've never used .6 since. What make is your welding machine ? I've welded Italian scooter bodywork with mine and that is thinner than most cars.
I switched to 0.8 when my old welder refused to feed 0.6 reliably in about 1990! I haven't had any problems on car bodywork with it & its very convenient when thicker metal needed welding. My new mig would probably feed it reliably but i can't be bothered to change over.