I'm using Michelin Pilot A/S Plus on my SAAB Aero as a summer tire and they are pretty amazing. I drove in one winter storm, not a lot of snow but very slippery, and was very impressed with their winter capabilities. The A/S 3 is indeed supposed to be the unicorn of tires (or holy grail if you prefer).
After decades of turning my nose up at all season tires for summer use I think Michelin may finally have cracked the code. I also fit a set of Michelin Pilot Exalto A/S to our Alfa Romeo 164 (the A/S plus didn't come in a 15 inch size, yes I said 15 inch, the Alfa is a 91). They are better than summer tires of a decade ago.
Trouble with high performance summer tires in a cool summer climate is you rarely get them up to temperature to deliver the ultimate grip you paid for, unless you push them hard routinely. Trouble with all season tires is they never deliver the grip you want, until maybe now. No other manufacturer has yet pulled off this neat trick although I'm sure they're not far behind. They all probably rushed out and bought a set of these new "magic" tires and are tearing them apart as we write.
You won't notice any difference from the two widths you are considering. 20 mm is less than one inch width.
Contact patches are always the same size for a given vehicle weight and tire pressure. Only the shape of the contact patches changes.
The dynamics of wider tires differs a little from the narrower taller tire but the ultimate grip is almost the same.
The 225 will be slightly lighter and deliver a slightly stiffer ride and sharper turn in. You'll run the 245/40 a little under spec pressure for the 225 to get your contact patch flat. Remember that the 225 will "fill the rim" better in that it is close to the upper recommended rim width ( which is 8.5 inches) and that the 245 will roll over a bit more easily in hard corners due to the wider tread on an 8.25 in rim, closer to 9 would fit the 245 better. This extra sidewall bulge actually dulls the handling.
The two tires are within 1% of each other in rolling circumference:
http://www.rimsntires.com/specs.jsp
And both are within 1-2% of stock rolling circumference so no issues there.