Part of the decision depends on how long you're planning to keep the Clarity. If it's a short-timer (3 years or less), just get a set of Michelin X-Ice in the stock size and do the seasonal switchover. If you plan to keep it a long time, an extra set of rims would be easier to do the semi-annual trade out. Size does not matter so much as getting the winter tires. Don't sweat the small stuff and just do it. Extra rims can be had at discounttiredirect.com if you go that way. As a personal preference, I don't like the narrow 45 series tires and wish the Clarity came with OEM 235/50-17 tires. But, I did buy a set of the 235/45-18 X-Ice last year and love them as a winter tire. I'm about to do the winter change-over at my local tire place which does it for free because I bought the tires there.
And, by the way, use the search term winter tires on this forum and you'll get hours of reading and opinions on this topic!!!!!