0.6 is better for welding on a car. I.E. 1mm.
0.8 for thick stuff.
As for brand- buy decent stuff from your local welding supplier, don't use rusty wire and all will be fine. Welding wire is welding wire, I have found no difference when changing from one brand of wire reel to another.
Technique, work piece prep and how your welder is maintained has far more bearing on quality of welds.
It's very much personal preference, and how important factors are like feed quality, and what you intend to weld.
We use Nexus or Esab wire depending on what we are doing.