What is the difference in the ride quality between 17" and 18" wheels? Or does the tire you choose have more impact on that?

 

http://www.miata.net/garage/tirecalc.html

Great tool to size up different configurations.
If you kept the diameter the same after going to 18s, the sidewall would be smaller resulting in a harder ride. Larger potholes will damage or break your wheels. If you increase your tire size diameter proportionally, the ride will be a little softer but you'll experience a decrease in MPG at lower speeds and slower acceleration all through out. Not to mention added unsprung weight which will increase braking distance, rotor heat, rotor warpage and braking distance.

 

yeah its really how much sidewall you have on your tires. and the tires of choice. eg. if you got a 235-40-18 vs 235-40-17 in the same tire the ride comfort will be similar because of the same sidewall height. the 1 inch difference in rim size will be noticable in the gearing. of course when you switch to a shorter sidewall tire, the ride will be more harsh.

tire choice has a lot to do with this too. some tires have very very stiff sidewalls, so when you decide on tires, this would be one of your priority criteria