The 18"s have a smoother ride but felt less connected to the road.
That's as much a function of the tire construction and air pressure as the relatively moderate difference in aspect ratio.
The sweet spot in terms of sidewall height very much depends upon the road surface and the suspension tuning of the vehicle. The two go hand in hand (along with tire construction). I like my vehicle to shine in all commonly encountered conditions. Unless you will only be on perfect pavement with no mismatched expansion seams, over-lapping blacktop layers, occasional small stones/gravel, etc., I do think the 17" wheels with their 4.5" of sidewall leave very little on the table and is definitely better for unpaved roads than setups with less sidewall. You could likely get a little more out of a good 18" or 19" setup on smooth pavement but the difference would be small (assuming optimum tire selection in all setups) and performance on unpaved roads suffers. This is why rally cars run tires with generous sidewall heights.
As the tire volume decreases (as is the case with the lower profiles), a corresponding increase in tire pressure is required to support the vehicle. That is why models with 19" wheels specify two PSI higher. All else being equal, this necessitates running a slightly softer compound to achieve similar cornering forces.
Matching all of these things up is very much a guessing game although it does become less hit and miss as you gain experience. This is why tire reviews should not be taken too seriously unless they are done on the same car in the same size tire. Using the latest snow tire I purchased in 225/65/17 as an example, the Goodyear Ultra Grip Ice WRT for passenger cars (not the SUV model). Tire Rack tested this tire and three other winter tires on a BMW 328i sedan with 215/60/16 tires. Not only is this a narrower tire than the CX-5, it also has about 12% less sidewall. I've done 3 winters on it and driven it hard on plenty of bare or bare/wet pavement. My first impression was that it had exceptional steering response and under hard cornering, to the limits of traction, it was very predictable and had no tendency to "tuck under". It was a little better than the stock Geolanders and far better than any winter tire I had driven. However, Tire Rack testers rated it very low in handling dry and wet pavement, the exact opposite of my findings. I felt the sidewalls were very supportive and I think in the lower profile used on the BMW, the tire was too "wooden". But on the CX-5 it performs brilliantly. They also claimed it had the loudest roar of all the winter tires in the test but I found the sound levels very tame, about on par with the Geolanders. Again, the supportive sidewall may transmit more road noise to the wheel but, in the higher profiles, maybe this noise is dampened sufficiently.
The point being, the tire has a lot to do with what is the perfect sidewall height. I find the tire selection in 225/60/19 to be so poor that I would eliminate the 19" wheel option on that basis alone.
As an aside, the Goodyear Ultra-Grip Ice WRT is finally available again in 225/65/17 (after being completely sold out last year). The new production is just arriving in shops. I only mention this because I've found it to play exceptionally well with the CX-5 in the OEM 17" size and is a great choice for anyone who does more miles on wet/cold roads than ice/snow and wants a tire that retains the sporty handling of the CX-5 while still handling the slippery stuff with authority. Surprisingly, it is also very affordable at only $115 at Tire Rack.
