Originally Posted by SleepTight
It is simple physics. The wider the tire, the wider the path it has to "plow" through deep snow and the greater chance of losing traction--particularly at the rear wheels in the M5. I would recommend the standard M5 19" wheels for winter duty not because of their smaller diameter, but because of their narrower widths of 265/40 R19 front and 295/35 R19 rear.
The optional M5 20" wheels are 265/35 R20 front 295/30 R20 rear.
It is possible that some other 19" wheel with an even narrower width will also fit the M5, which (assuming that it didn't radically upset the M5's delicate suspension) would be an even better choice for driving in snow, but we know for sure that the standard 19"s will fit so that's probably the way to go for now.
Im assuming OP is referring to the wheel size with a winter snow tire in mind as well. If so then the width of the stock tires would not matter. However, Im assuming the 19" wheel is more narrow than the 20" wheel, which would allow for a skinnier snow tire, which was your argument.
Also, I'm assuming with a tire that has a larger sidewall by an inch would provide a deeper tread, aiding in snow traction. In that case I'd say 19's are better.