What are the benefits for having 18" over 17" wheels? My present configuration is the 18-incher, but I have learned that the run flats run considerably harsher with the larger wheels. I had a loan car with 17" wheels and run flats and was surprised at how "normal" the ride was.

 

Mostly it looks better to many people.

If the overall diameter of the tires are the same, you have less rubber sidewall. That's bad for ride quality, but good for precision and (lateral) grip. (Meaning, better cornering--slightly.)

Tires in smaller sizes are becoming harder to find for some applications. Not 17s, at least at this point, but certainly 14s are a real challenge. And 15s are somewhat of a challenge as well. If the trend continues, your choice of tires for 17s may be a little limited in a decade or so.

The 17s should (all other things being equal) ride better than the 18s. The wheels and tires also tend to be less expensive, which is also a plus to me.

But, hey, I like the 15s on my car!