Need Winter tire/wheels advice
 

I just purchased my first Lexus, a 2017 RX350. I got it with the optional 20 inch wheels and Bridgestone Ecopia H/L 422Plus tires (235/55/R20).

I'm concerned that these tires will not do particularly well in my Utah Winter climate. I would like to buy new wheels and Winter tires. But I'm not sure what I should buy. What are the pros and cons of different size wheels for my snow tires? My local Discount Tires dealer says that I can fit either 17 inch or 18 inch wheels on the vehicle. He strongly advises against the 17 inch in favor of the 18 inch. But I couldn't really understand his reasoning. I thought 17 inch wheels would allow for tires with higher sidewalls and better performance in snow.

On what should I base my decision on which wheel size to go with?
Thanks.
John