Just go outside and find, so many Myvi's running around. If you care so much about comfort, go for 15", and the tyres are cheaper as well. 1" won't make so much of a difference. You can use the money saved to do something about your suspension etc.

Its not about the size, for me its more about the design. Some small rims can really complement the car, while other "huge" 18" rims messes with the car's balance and image. No offence, but when you look at the Myvi, you will see that the fender distance from the bonnet is quite huge. No matter how big of a rim you use, the car still looks some sort of like a potato. For better effect, I think lowering the car will enhance the sportiness more.

It will still roll around without better suspension....if you want comfort, 15" rim is better cause with 16-17" rims you have to use lower profile tyres. For comfort, why not you invest the extra money for sound proofing or chassis stiffening (autofoam etc).