I'm not a welding pro but I think MIG's better for thin stuff as you have more control over it and can get a better weld. You can weld thin stuff with arc too but if you have the current set too high you risk blowing holes in the metal.

Arc is high penetration compared to MIG so yeah, it can weld thicker stuff pretty well.

I'd say go for arc. I've had Gary's old welder for a few years now and it's still going great. I don't weld regularly enough to get really good welds but I can get by for robot stuff. Barely spent anything on consumables in that time and I don't have a tank taking up space in the shed. Plus since arc's harder to pick up, you'll probably find it easier to transition across to the other types, something that might not hold true if you learned on MIG then had to switch to arc. But like I said, I'm no pro.