Well, it depends.
I agree that 19" will change appearance more.
But still I would say that go with the M3 18".
Bigger rims and lower profile tires will make the car look little bit goofy, because then the wheelgap will usually be so noticiable and will need lowering.
And also M3 wheels are safer option.
I don't know about those M6 19" wheels and stock suspension, will there be how much clearance.
At my case when I have lowered the car quite a bit, rear tires were rubbing to fenders, so I had roll them to make a clearance.
Front tires are rubbing to inner wheelwell when tires are fully turned.