If you are running the same size tires, you should generally run the same size wheels. 235/40R18 will fit on the 9" wheel, but there is really no good reason for doing so for winter. The wider rear wheels/tires will look better, but for actual snow performance, narrower is better.
Staggered setups make sense for those with milder winters who want clear-road handling more similar to what they have in the summer.
The Tire Rack is gradually completing the winter database for this car. Until then, most reps will (rightly) stick to the most "standard" configuration, which is the staggered setup. That is the same thing I would do if it were another car that I was not specializing in.
Since I work with this particular car every day, I have a bit more information than average, and don't mind going to things that are BMW recommended but have not made it into our database yet. If you have any questions, always feel free to call me at the number in my signature.
I have accepted a new position at Tire Rack and am no longer in the sales department. email@example.com
is available to assist with M3 questions.